Michael, Michael, Michael….if you're gonna place NX-Tremes in your room, you'll want a matching set of OB servo subs to go along with them. For home theater, the SVS Ultra is probably fantastic, but for music, it will be a weak link.
Early B, I have a single SVS Ultra sitting along the right wall. I have space on front wall for a second SVS Ultra if needed. Give me a reason. I really like this sub.
If you want to run with Early B's recommendation going with OB servo towers, you would probably want to flank your main speakers and run those as a stereo pair. If I'm remembering correctly, don't the NX-Tremes play pretty darn low with all those drivers? Sure, OB servo sub towers are great and all, but I have to wonder if swarming a group of sealed subs wouldn't get more or less to the same place if one was crossing over very low, like 30-40Hz?
Michael, as to giving you a reason to pick up another sub, adding another sub (or two) would help smooth out the frequency response down low in the room/at the listening position - two should end up better than one, and three is what seems to be the research consensus on what makes up a minimal "swarm" arrangement, which with your setup having one on the right wall, one (proposed) on the front wall, and then maybe one more along another wall wherever asymmetrically it might fit would be about a textbook implementation of a "swarm".