Daev’s Lab Notebook: Bose 901 Series VI journey

(incept date )

My Bose 901’s

I inherited my Dad’s beloved Bose 901 series VI speakers, along with the correct series VI “Active Equalizer” box. These are very polarizing speakers in the hi-fi world, so I’m curious to see what I think. I’ll restore them as needed back to factory spec, and then evaluate the sound for myself..

tl; dr:
These speakers are quite picky about room placement and acoustics, so your experience may differ wildly from mine. Or any else’s, for that matter.
When set up properly:
– These are terrific as low to medium volume “background party music” speakers, where the diffuse sound field nicely fills a room without excessively loud hot spots.
– Very engaging for symphonic, big band, and arena rock. The big sound field adds a lot of “space” to the presentation.
– They’re not my favorite for chamber music, jazz, and solo vocalists. The “large” sound field made instruments seem too large, and a single singing voice was oddly wide.

OK, let’s get started!

First, room placement!

BTW, here’s my real world, “best compromise” positions.

The “Active Eq” processor box

As an experienced old techie, the first thing I look at are the electronics (in this case the “Active Equalizer” or audio processor).
I look to see if I need to fix or rebuild anything, I check to see if it still meets factory specs, and I listen to it.
This example actually appears to be in decent shape, despite it’s years. THD and noise is right where the service manual (and another reviewer) said it should be, with no visible or audible signs of bad caps or other components. The EQ curve seems about right as well.

Here’s a measured graph of the series 6 processor box’s EQ curve with controls set to the detent.

– Per Bose documentation, the reference unity gain point through the EQ is at 3200 Hz, so that’s the “0” dB ref point for this graph.
– Measured with 500 mV input, THD is good in the normal audio range, below 0.05%.
– But check out those huge eq boosts and cuts! That weird wrinkle in the 1.5 to 3k range is odd, as is the one at about 350 Hz. This complex EQ curve can’t be fully emulated with out a parametric EQ, a simple graphic EQ isn’t agile enough.
– Lastly, the huge bass boost and then abrupt falloff at 35 Hz (12 dB/octave) suggests crossing over to a subwoofer around 80 Hz would be a good idea.

Note: The pro PA version of this unit, the 802, actually has some published specs. A couple of interesting ones:
– Recommended a high pass filter at 55Hz (12dB/octave). This tracks, although I usually start with 80Hz (the THX standard) for general subwoofer use.
– Bandwidth is rated from 52Hz to 15kHz

The Speakers

The low speaker sensitivity (series I and II 1 appx 81 dB, – 2.83V @ 1 meter, series III and later closer to 88 dB) along with the later generation ported design suggests a fairly high power amplifier with a high damping factor will yield the best results with these speakers.
Indeed, the series II owners manual recommends 100 watts or more for large rooms and a damping factor of 40 or more.

I have several amp candidates
– A QSC PLX 3102 rated at 600W (8Ω), 1000W (4Ω), and 1550W (2Ω). And that’s in 2 channel mode! This drives the garage/PA system.
– A vintage Hafler Pro 500, appx 250 watts/chn – the beast!
– A vintage Carver PM 300 in a handy 1U rack chassis, appx 150/watts chn.

2023.11.02 – Speakers arrived at my house. Excited unpacking noises ensue.

The date on the speaker terminal cup label is 1987.

A quick buzz and rattle frequency sweep didn’t turn up any issues either, so I’ll do a little cabinet cleanup and then off for quick listen.

The M.C.M. tulips look retro/cool, but they have… issues.

• They ring like crazy when tapped. That can’t be good.
• They are way too short for my seated ear level.
• They are rather unstable on carpet, especially around my large, enthusiastic, parkour inclined cat.
• They attach to the cabinet bottom with annoyingly cheesy wood screws, which make the cabinet wood crumble and is horrid way to treat speaker cabinets. And this weird mounting schema also gets in the way of the speaker connections on the speakers bottom side. It’s all just too meh for me.
  So:
• I’ve ordered some PA tripod speaker stands. These stands are adjustable vertically and they have a much wider tripod stance for better stability.
  I’ll modify the stand platforms by punching a 1″ hole for the speaker wiring, and then drill four holes to mount the plate to the 901 speaker securely. To do so, I filled the tulip stand mounting gouges on the 901’s cabinet bottom side and then installed four threaded inserts to attach the stands. I used “E-Z Lok Threaded Insert, Hex-Flush, #10-24″ using 1/2” #10-24 hex head screws with washers. Works a treat!
• Since I’ll likely forget to be careful next time I separate the speaker and stand, I’ll rig up a safety disconnect with banana jacks and plugs for the speaker wiring.
• Lastly, I’ll put some solid rubber feet on the cabinet bottom to avoid scuffing it further, to handle shelf/stage placement situations, and to avoid squashing the wiring and my fingers when used without the stands.
• After putting all this together the speaker stands were still too wobbly and made a creaking noise. Ugh! After some thought, some large steel washers for strength and and rubber washers for the creaking alleviated these issues.

It’s 2024, March: I finally had the time to rearrange my living room to accommodate the 901’s. Turned out the PA tripod stands are too industrial looking and cluttered the room up too much. So the 901’s just went on top of the Heresy 3 speakers, after swapping the Klipsch wooden tilt wedges for steel stands instead.
Looks much better, and better room placement for both speaker sets!

First placement, appx 10″ from back wall, toed in slightly.

When you live in a small, shared space place, compromises must be made.
The 901’s speakers are about 10″ off the back wall, in line with the official placement recommendations. Distance between and distance to listener are good as well. Side wall distance is a bit asymmetrical, but “it is what it are”. After some listening, I wound up toeing in both speakers sets for best imaging in this space.

Initial 901 listening impressions:
Plus’s:
+ The broad sound field works nicely for larger recorded spaces, and can really open up older congested or dry recordings, like the remastered Glenn Miller catalog. And classic arena rock (think Pink Floyd) is great fun!
+ Club and trance has good punch and energy.
+ Ambient, chill, and new age are all rendered pleasantly in a diffuse sound field.
+ Mickey Hart’s “Drum Planet” is a standout! Transient response makes difference!
Minus’s:
– Chamber music is presented with lots of air, but the instrument presentation felt unnaturally “wide” for the perceived distance.
– Closely mic’d “vocalist forward” music sounds too distant, and that recessed presentation has a lack of placement precision and focus. Linda Ronstadt’s voice is about 4 feet wide in my room, but she sounds about 20 feet away.
These latter two types of music are reproduced much more to my liking on the Heresy’s or Genelec’s in this room.

Other speakers in photo:
-The mini monitors on the 901’s and room center are Genelec 8010A’s mini monitors, used for my home theater.
– The floor speakers are Klipsch Heresy 3’s driven from either a tube amp (usually a pair of Dynaco Mark IV mono-blocks) or a Hafler SE-120 MOSFET amp.

Update 2024.05.12
Picked up a second set of 901 series VI to experiment with, but they were in too nice a condition so I’ll avoid doing anything that would trash them. Instead, they became the garage/photo studio speakers.

Update 2025.11
Picked up a third set of 901 series VI in rough cabinets. These will be my experimental, PA, and occasional portable units. Looking at package straps as a non destructive way to put handles on ’em.

Things to think about / try

• Use with a dbx drive rack PA2
  See how well a dbx drive rack PA2 can be used to effectively replace the Bose processor box. The extra headroom of the pro level +4 dbV signal path will be helpful here, especially when fed to the “pro” QSC amp. Use the output PEQ (+/- 20dB parametric) to first cut/flatten that large midrange hump, then use the Auto PEQ as usual to trim up the overall in room frequency response. That leaves the front end GEQ (+/- 12dB GEQ for touchups and house curves.
  
  -Update 1: This works quite well! I did a basic flatten of the 901’s using the post crossover PEQ with the built in RTA with pink noise, closely mic’d.
  Then I ran the room correction pass to handle the room nodes and fine tuning. Here’s the initial settings I used for the PEQ:
  Band 1: 450Hz, -10dB, Q1 – Broad midband cut to flatten out the midrange hump.
  Band 2: 70Hz, +6dB, Q2 – Bass boost for bass roll off
  Band 3: 15kHz, +6dB, Q1 – Treble boost for treble roll off
  
  – The dbx built in RTA is quite useful, as is the complete control from a PC or iPad via the usual app. The output PEQ does the bulk of the heavy lifting to emulate the basic 901 eq curve, the crossover handles the handoff to the sub with aplomb, the room correction is fast and easy to use, and the front end graphic EQ can easily tweak for source or personal taste without disturbing the calibrated speaker EQ’s. Win.
  
  -Update 2: Per Jim Weir, how to use a four band parametric EQ to Mimic the Bose EQ.
• Note: The extremes seem a bit aggressive to me. It probably incorporates the Bose “House Curve” along with the Speaker EQ, see the 802 pro series curve for reference.
  Band 1 is a 1.4Q +12dB centered on 35- 40Hz,
  Band 2 is a 1.4Q +3dB boost at 100-110 Hz,
  Band 3 is a 0.4Q -5dB cut at 400Hz,
  band 4 is a 0.8Q +15dB boost at 15K
  
• Subwoofer
  I’ll bet these speakers will be much happier with a subwoofer, based on that huge bass boost in the processor and the amount of port chuffing I hear below 50 Hz. This is also trivial to do with a dbx drive rack PA2, yay.
  UPDATE 1: Yes, this works really well. The sub handles the deep bass chores, freeing up the 901’s to let their fast bass “snap” really come through. I used an 80 Hz @ 24 dB crossover point.
  
  • Supertweeter
    A more radical idea: Since a 4″ driver isn’t the best for high frequencies due to mass and beaming issues, I’ll try adding a dipole Linaeum super tweeter on top (from Optimus LX-5s) with the dbx active crossover at 8 kHz. I’ll try to acoustically pad down the front lobe to blend with the 89/11 ratio, and also sound dampen the top of the 901 for reflections. The LX-5 Linaeum was crossed over at about 5K with a simple 6dB bass blocker series cap of 6.2 uF, and I’ll keep that value as a tweeter protection device.
    UPDATE: The Linaeum tweets indeed add significant sparkle and better dispersion on the high end, and I’ll bet the imaging improves without destroying the lush enveloping sound. My instruments say the high end is smoother and has better extension. My ears say transient response of triangles is better and there’s more “air”. But it is another amp and pair of transducers to wire and maintain…
    
  • PA using backwards 901’s and dbx drive rack
    This idea is to flip the 901’s around and using as a small venue “802 type” PA speaker, only with a built in mini stage monitor. 🙂 I note the first tests of the 802 PA concept was done with production 901’s, so not a new idea.
    
    I occasionally host dance shows where the dancers need to hear the music on stage clearly, and this reverse application might allow me to quit hauling around separate monitor wedges.
    
    Interestingly, Bose recommends these Parametric EQ settings for the Pro series 802
    Band 1: 80Hz, +4.0dB , Q1.4
    Band 2: 160Hz, +4.0dB, Q1.2
    Band 3: 300Hz, -4.5dB, Q1.0
    Band 4: 619Hz, -5.7dB, Q1.5
    Band 5: 979Hz, +1.2dB, Q1.38
    Band 6: 1322Hz, -5.6dB, Q1.0
    Band 7: 2845Hz, -0.5 dB, Q1.0
    Band 8: 6200Hz, +5.2dB, Hi shelf
    
    This 802 adjustment curve has considerably less boost at the frequency extremes than the home 901 curve I measured, which leads me to believe the 901 active equalizer curve also incorporates the Bose “house curve”, probably a smiley face EQ. This will be ferreted out in due time.

Other pie in the sky projects

• 2024.12 – Current Production Compatible Receiver search
  Look for some current run integrated amps / receivers to pair with the 901’s
  
  NAD C3050 LE w Dirac module.
  It looks like it has a fairly muscular power amp, and also some sort of subwoofer output jack, and it has a pre out / power amp in loop, which would work fine for the active EQ.
  But even more interesting is the possibility of using the Dirac module as a combined eq replacement / room EQ correction module. (It’s also possible the Dirac room correction will be severely confused by the 901’s reflection technology, will need to test this feature.)
  
  Outlaw Audio RR2160MkII receiver
  Has a processor loop and pre out amp in jacks.
  
• Investigate the Deer Creek DSP boxes. They have a series with the Bose EQ pre-programmed in, looks like a good replacement for a missing EQ.
  
  

Last Updated on 2026-02-24 by Daev Roehr