re: STF-1 vs. EP175 -- it's tough to say. Bigger is better when it comes to filling a large room with bass, particularly if you are into shaking walls and furniture. On the other hand, a smaller woofer in the same size box can often do a better job of going down into the lower frequencies. The STF-1 is a surprisingly good but smaller subwoofer; it may be "better for the size" than the EP175 (in fact it probably is) but I don't think it would be better overall. Having said that, unless you have to shake the house I think you would be real happy with a matched Axiom system and an STF-1.
But what do I know ? Until recently I was running M2 mains, VP100, QS8s and a big honkin' SVS 20-39PC-plus. In all seriousness, if you expect to become a bass addict one option would be to go for smaller mains (M2 or M3) and spend the extra on a bigger sub.
The more likely scenario, however, is that you will play some music on the system, be blown away by how good it sounds, start digging out all of the CDs you haven't listened to for a while, and be real glad you got M22s and the matching surrounds (in which case the STF-1 is probably the perfect purchase).
It's hard to say, every addiction plays out a bit differently
