I run duel Rythmik 15 SE's....for 100% HT.
I used to Run a single SVS PC Ultra, and several HSU's but not the ULS-15.
Needless to say, the rythmik's won out, even 1 of them is about even with the svs, bass quality and tightness is unmatched even when up against the SVS. The Rythmik actually goes deeper more evenly, the SVS goes a bit louder, but it really depends on your room.
I went with 2 and won't look back anytime soon, unless I just get bored or somehow become rich.