You could take a look at one of the Q series Rel subs. I don't have any extended experience with one (only at an in-store demo). Sounds clean and a fair amount of adjustments. An older Vandersteen 2w (non q) would be fairly close to your budget.
The Onix Rocket UFW-10 is hard to beat in a small to medium sized room. It won't extend into the 20s but does very well into the low 30s. But most importantly it is tight, articulate bass, not boomy HT type bass. It's s little over your budget at $600.00, but well worth if it you can stretch.
Given your budget, you may want to consider the Outlaw Audio subwoofers, which were designed for them by Hsu. They have gotten good reviews, and offer a lot of performance for the money. Click on the following link to Outlaw to learn more: http://www.outlawaudio.com/products/lfm1.html
I use the Q108E REL with Von Schweikert VR-1s in a small room. Nice and small, bass to 30 Hz. To get lower you will need a larger sub. I also have a beautiful Hsu VTF-3R that goes to 18 Hz (-2 dB), flat to 20 Hz., but it is bigger. Great price and great service.
Bob, I think that disturbing the signal to the mains with a high pass filter or external crossovers usually introduces undesireable grain in the mains-- short of rather high $ implementations. There is no problem with bass augmentation mode with LFP in the sub as used by REL or Vel DD-- provided the system is set up carefully.
The Rel Storm 3 that I own has been set up quite meticulously, and the crossover is set at 27 hertz. It integrates quite seamlessly with the mains, which run at full range. It augments the lowest of the low frequencies but apparently does not take any bass load off the mains. The result in my system is excellent, for now.
Bob, it is not apples-to-apples to compare the low distortion products of op amps-- which nevertheless at higher frequencies add audible grain & hardness & loss of refinement in the treble and midrange-- to the much higher percentage distortion observed in bass drivers-- which is often perceived by the ear as euphonic warmth. Op amps in the analog domain can be good or bad (usually bad). My experience has been that even a good op amp chip is highly sensitive to power source. It's rare at even higher price points to find discrete voltage regulation or clean battery sources applied to solve the problem. Not to mention possible inductance/capacitance issues associated with long IC runs to/from the sub in a configuration that utilizes the sub's inboard HPF.
Since early audiophile subs like the Muse came on the scene in the mid-90s, reviewers have consistently observed the hardening of treble that can occur when adding a sub's inboard HPF in the chain to the main amp. There are no absolutes on these points. Doubtless some main speaker/main amp combos benefit more than others from an inline HPF that relieves them from handling bass frequencies. But there is no need to introduce a trade-off between treble & bass performance, if the main amp/speakers are well matched. Short of spending $1000+ on an outboard Marchand or equivalent xover, I would advise sub users to stick to bass augmentation mode. Whether this is best delivered via speaker-level inputs or line-level inputs to the sub is another debate.