Subwoofer Decision


I have narrowed my search to two choices though I am sure I will get comments steering me away from these.
Contrast Rythmik Audio 12G with Vandersteen 2Wq. I would like to get only one though I know a pair are best choice. Could maybe afford 2 Rythmik, but only 1 Vandersteen for now. The subwoofer would support Dali Helicons (4 ohms) biwired and driven by 2 mono McIntosh MC 252's at 500 watts each into 4 ohms. Living room size aprox. 24'x16'x8' placement not centered on 16' wall due to furniture (wife) constraints must stay put. Subwoofer placement needs also to stay there as well, I know this limits possible best choice for placement, my bad. There is room behind, next to, and between speakers. Any help help is appreciated.
128x128lowfreqguy
I do not have the level of experience as many of the people that have posted above.  However, I decide to post a response because I have used SVS model sb1000 with several sets of speakers, amps, and preamps. I only use one sub. I have been able to adjust the sub in each situation to get the sound I wanted. I have used tube gear (AR, Lamm) and solid state (Pass).  They provide the buyer with a try-out period. Great little sub for a high-end system. 

As another indicator of the sound characteristic of the Rythmik F12G sub, one listener's reaction upon hearing it was that it sounded "leaner" than other subs. To me that's a compliment!

Hartf36 mentioned the Rythmik's adjustability. Brian Ding considers the phase matching of a sub with it's partnering loudspeaker a major priority, and his phase control is the best I've seen. It is a continuously variable rotary knob that allows the close alignment in time of the sub's woofer with the speaker. The sub's position in the room can be selected with the matter of the room's modes (the locations in the room where low frequencies resonate the most---a function of the room's dimensions, causing boom and overhang) as the priority, the phase between the sub and speaker then adjusted via the sub's phase control. Most sub's force one to choose between the two.

When a sub is not precisely aligned with it's speaker, not only is the sound quality of the pairing reduced, so is it's output. With the sub and the speaker slightly out of phase at certain frequencies, their opposing polarities cause a cancellation of combined output at those frequencies. The Rythmik phase control allows the minimization of that phenomenon.

A fair number of the contributors on Rythmik's AVS Forum are Magneplanar speaker owners; the subs are finding favor with that and other planar speaker owners.  

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!)