The 8 or 10 inch Velodynes look like they might work well. Have you heard either one by any chance?
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I don't know if your application is perfect for a sub. IMO the best use for a sub is the bottom octave of bass that your main speakers won't produce. 70 Hz is pretty high for a sub where your ears can easily locate it. I had some minimonitors that went down to about 55 Hz and no matter what I tried it was difficult to locate the sub so that you couldn't hear it as a separate entity. I now run my Wilsons full-range and just use my REL sub to produce the lowest bass, i.e. to pressurize the room. I find the best way is to roll off the sub with its low-pass filter and connect the high-level sub inputs to the speaker outputs on my amp. Maybe if you locate the sub near the main speakers, or better yet, buy a pair of subs and locate them near your main speakers.
Okay, I have some rather unorthodox views on bass reproduction, so let me just toss this out and see where it lands.
The problem with bass reproduction in small rooms is not that there are too many peaks and dips - the problem is that there are too few! At midrange and treble frequencies there are many many in-rooms peaks and dips related to reflections off of the walls, floor, and ceiling. They end up being so close together that none of them stand out to the ears. In the bass, the peaks and dips associated with room reflections are spaced far enough apart that they are audible.
You see, the ear/brain system integrates sounds over a bandwidth of roughly 1/3 octave. If there's a definite peaking or dipping trend over a third of an octave, the ear will detect it. But if the peaks and dips are close enough together that they tend to average out across a 1/3 octave interval, the ear doesn't detect them.
Bass tends to sound more natural in large rooms than in small rooms because the room-induced peaks and dips are more plentiful and spaced closer together, and therefore tend to be less audible. The term that describes the bass energy in a large room is "decorrelated" - that is, it's randomized and doesn't over- or under-emphasize any particular region. In a small room, the in-room bass energy tends to be "correlated" - that is, the direct and reflected bass energy combines in a clearly distinguishable pattern of peaks and dips. We can move these peaks and dips around by moving either the woofer or the listening position, and some combinations of peaks and dips will sound better than others.
Equalization can approximate the decorrelated bass field of a large room by bringing down the peaks and bringing up the dips. Unfortunately, equalizating for smooth response in one location can and usually does make the response less smooth in other locations. This isn't an issue if you stay in or near the sweet spot, but is if you want good quality bass reproduction throughout the room.
There is a technique for de-correlating the bass in a small room so that it more closely resembles what we'd get in a large room. The technique is this: Use multiple woofers scattered around the room. Each woofer will produce its own unique location-dependent peak-and-dip pattern, and combining a number of dissimilar peak-and-dip patterns tends to smooth out the in-room bass at all listening positions.
The technique of using multiple subs is advocated by Todd Welti. While he examines symmetrical placement in his paper, the same prinicples apply. From his Audio Engineering Society convention paper #5602, presented in May of 2002:
"...[W]ith enough subwoofers, it is theoretically possible to cancel out all modes in a room."
So more subwoofers is beneficial... but we can't use some ridiculous astronomical number of subs. How much benefit do we get from going from one to two subwoofers, or from two to three?
Physicist specializing in small-room acoustics and loudspeaker design Earl Geddes wrote this in an Audio Asylum post on October 30, 2005:
"The spatial variations, and to a certain extent the frequency response variations, will go down (get smoother) as 1/N, where N is the number of independent sources."
So the answer is that most of the gains are early on, as you go from one to two subs, or from two to three or four. The improvement in going from one to four is as great as the additional improvement in going from four to sixteen.
Finally getting back to Cspaldi's question, I suggest as an alternative to a single uber-sub getting two or three or four small but still high quality subs, and placing them asymmetrically around the room. Now you will definitely get deeper and louder bass from a single sub for the same amount of money. If you decide to try the mulitple-sub route, look for subs with a fourth order low-pass filter because being able to sharply roll off the sub's upper frequency output is important for giving you good placement flexibility - with the more common second order low-pass filter, it will be easier to hear the subs' locations.
Let me know if you have any questions.
Best of luck to you,