Subwoofer Decision

I forgot to mention the speakers I have used with the sb-1000 sub. Magnepan 1.7, YG Carmel, Quad 2805, and YG Kipod 2. The sub blended perfectly with each of these speakers. I should also mention that I listen mostly to classical, especially symphony music. 

I can second the Hsu ULS-15 MkII.  I run a pair and they offer very tight clean bass down to 20hz in my room.  A great value.  I suspect either of the subs you are looking at would be great as well.  In the end what's "best" would only be known after an audition in your room.  That's the tough part--especially with subs. (thinking back to my aching arms after moving the Hsu's all over my room finding the best location!)

If you like Rythmik... you can step it up a notch with a minor price increase from Salk-Sound.
http://www.salksound.com/model.php?model=Rythmik%2012%20Subwoofer

Agree that the room is your biggest issue when trying to reproduce very low frequencies, but having a system that at least has the capacity t do that is also needed.

I have usually had speakers in my main system with pretty good low frequency reproduction. My Vandersteen 4As were the predecessors to the 5s with integral subs - the difference being that the 4s needed separate amps and external crossovers I have also use Vandersteen 2W subs and liked them.

I currently have main speakers that are 3 dB down at 20 hz, which covers just about everything and the replicate realistic 'slam' of a live performance quite well.  I run a pair of Hsu VTF-15 Mk. 2, their current top model, exclusively for bass reinforcement in home theatre use. I don't use them for music, but once did leave them in the loop to do some listening. Run the way I run them, they are 3 dB down at 16 hz.  Is that important? Normally not, but I was listening to digital recordings of European organs with material down to 16 hz (it is important to note that IMHO, you do not hear material that low, rather you feel it).

For the special purpose indicated, the very low bass reinforcement was effective and seamless - I think these particular subs have a very good chance of mating with decent but slightly bass challenged main speakers, and they have considerable adjustability

I certainly agree with wsphon's comments above. But I would also add that with my main speakers, well out into the room, that the Hsu subs have done a great job of recreating deep and tight bass found in many of the recordings (classical and other large scale works) beyond just my organ recordings.  That's what I like about subs in part.  You can optimize your mains for sound stage and the subs for deep bass.  Part of the success of these subs is the huge custom bass trap I made (see my system pics) which really helps bass quality. No bloat.  Just tight and deep with great impact.