I thought I'd share my experience with a used but newly bought (on eBay) Rane PE-15 parametric equalizer. New $300, got it for $100 incl. shipping, and it has only cosmetic damage that's irrelevant to me since it's going under the furniture, right beside the sub amp.
The Rane PE-15 is an analog parametric equalizer. It has 5 bands, with overlapping center frequencies of 20 to 300 Hz, 60 Hz to 1 kHz, 150 Hz to 2.5 kHz etc. Each band has knobs for filter frequency, bandwidth (0.03 to 1.5 octaves), and attenuation/boost (-15dB to +20dB).
I got it to get rid of nasty +20dB room resonances at 50 Hz and 80 Hz. See, because of the WAF, my Hsu TN1220HO is hidden behind a 6' armoire in the corner of a 23x13'x9' room, and the listening position is right in the center of the room.
I was thinking of getting the BFD, but I figured that playing with filter tuning knobs is more fun than programming digital. So when I finally won a PE auction at a price I was happy with (took 3 auctions and 2 weeks of trying), that settled it.
Anyways, the PE-15 looks very industrial, reminds me of the Hsu 500W amp. I just plugged the darn thing between the receiver and amp (using RCA to 1/4" mono cables from Radio Shack), fearing the worst (ground loop).
Sure enough, I plug it in, turn it on and there's a very very loud hum. $*%&@! I say to myself, but then it occurs to me that this is a used box, and the cables are cheap, so after some mucking with the connectors the hum just went away. (Probably just some corrosion on the contacts in the PE-15, so it wasn't a ground loop but a floating ground on the RCA cable going to the sub amp!)
I used a downward bass log sweep from the Boston Acoustical Society test CD, and used the two lowest frequency bands, one at a time. I set the center frequency by eye to 80 Hz, the bandwidth to the minimum 0.03 octaves, and the attenuation to -15 dB. I then tweaked the center frequency until it seemed (by ear and by Radio Shack SPL) that the notch filter was set to the center of the resonance, and then I backed off the attenuation factor and widened the bandwidth until it seemed reasonably flat. (Room resonances don't boost uniformly within a room, and there are many common seating positions either when we're using the projector or just listening to music, so perfection in any one spot is pointless.) I then did the same for the 50 Hz peak. There, I had to rely on the SPL more because deep bass is kind of hard to pick up by ear in any quantitative way.
Then, because the resonances were gone, I could boost the bass gain in the receiver way up to keep the SPL flat from mid to deep bass. OK, there's still resonances but boy going from 20 dB to 6 dB makes a world of difference.
A couple of notes:
1. I was able to drop the receiver cross-over from 150 Hz to 100 Hz. Believe it or not, it did sound better at 150 Hz before equalization.
2. The PE-15 was passing audible noise to the TN1220HO when the filter bandwidth was set too tight. (They warn that happens in their manual.) After tuning it went away. Plus, I activated the sub amplifier's built-in cross-over because the noise was coming at frequencies that the receiver wasn't sending anyways. Finally, I reduced increased the sub gain in the receiver and reduced the sub amp gain to improve the signal-to-noise ratio.
3. This kind of product I'm pretty comfortable buying used. These kinds of analog circuits are bombproof compared to typical consumer electronics, so it'll either work perfectly or be burned out.
4. Previously, to keep the bass from being ridiculously boomy at the resonances, the sub gain had to be reduced way down, and while the sub dramatically improved the overall sound compared to just the mains (M3Ti pair in each front top corner of the room), there still wasn't much punch where there was supposed to be, like with drums, and pipe organ pedal tones just seemed weak. Lower bass music (bass lines in jazz say) would have audible distortions in volume vs frequency too.
In summary, I feel that adding the equalizer made as much of an improvement to the bass over the M3Ti+TN1220HO as the addition of the TN1220HO alone did. That's what bad room resonances do, they force you to back off the bass, so my deep bass (20-40 Hz) was maybe 15-20 dB lower than it is now. Now, it's deep, rich, nearly uniform bass to the depths.
Best $140 (incl. SPL and cables) I ever spent!