I have had my M3/M2/QS4 setup for about 8 weeks now. I'm using M3's as my mains, M2's for front and rear center, QS4 for surrounds. My living room (aka "the theater") is very open leading into the kitchen and study. I have stone tile floors. The room is approx. 16ft long by 11ft wide, with the left side open to the kitchen, and far end (behind the TV) opening to the study.
I put all this together thinking a sub would be required. Having run the specs on some different spare subs (originally for my car, just never used), I decided that I could build a sub as good as I could purchase. Especially since I just didn't have the $500 for a good powered sub.
With all this in mind, I've been playing my system using a variety of sources (lots of movies, though). I've concluded that I don't have to have the sub NOW. I'll have it built sometime hopefully by the end of Feb, but the M3's still sound great without a sub.
As for advice on what to get...
The M22's are a step above the M3's so I can't imagine them having less bass than the M3's. If you're handy with a screwdriver, you can buy a kit sub that sounds great for under $350(http://www.onhifi.com/product/dayton_titanicsub.htm).
If you're like me, you can use some left over stuff (a 12in. MTX sub, Onkyo 501 amp) and buy the parts to build one (sonotube, MDF, glue, screws, pvc pipe). I'm going after a sonotube design, but could nearly as easily build a cabinet design sub.
If you want the best sub for the buck (requiring no manual labor, cept to move it around the house) then I'd consider SVS first, Hsu second.
