20' x 20' is no small space. It should make for a great setup! I was looking closely at the M22 type setup of the Epic Grand Master 350 (only with a different sub at the time). I have an area that is essentially 12x30 with 8 foot ceilings. Your 20x20 room probably has 8 foot or greater ceilings, so you have a good amount of air volume to move sound through. I was given the "facts" from Alan from Axiom about air volume, and I ended up have to go above my budget and got a system like the Epic 60 only with the VP150 center, and again a different sub. This was a little more than a year ago. All I can say is that I am glad that I did. I really like the sound of the towers.
I know, you don't want something that "big" as you put it, but, and I know that Bose will tell you otherwise, but you can not truly create accurate mids and lows using only small speakers. In a larger room, the smaller speakers will get "lost" to some degree compared to larger ones. Basically, you just get a much fuller sound.
Now, of course, you probably have a budget to consider, and maybe spousal approval, but you would like the sound of the larger speakers, I am sure.
Plus, with a large display like a 72" unit, or front projector like Sirquack mentioned (I love 104" of viewing pleasure), the larger speakers will fit in just fine, and smaller ones actually may look more out of place if you are going for a visual thing....
I know some people have smaller speakers, like the M22 and love them, but just some thoughts.
