Welcome to the forum! Most excellent screen name, btw.
Congrats on the new gear, too! Some thoughts:
- Try running your Onkyo with a "phantom" center (by not using your center channel speaker, but rather having the two front speakers handle it), and then try it with the Sony center channel. You might find that you like it better without that Sony...or maybe not. Just a thought, as I'm guessing that those Monitors are much better than the HTiB speakers.
- If you can afford it, it never hurts to get a great sub. After all, you can always turn the volume down on a quality sub, but you can't turn the quality up on a junky one. Plus, when you move into a larger room, you're set.
- You can always contact SVS and get their thoughts on which sub would fit your needs/desires the best. I haven't corresponded with them much since 2007/2008 (back when I bought my sub), but they were always open and honest with me - and I've read the same from everyone else that I've seen. I don't think that they'd try to sell you more than you want.