my room is 16 x 24 and it's doing quite nicely.
You will probably be fine. What we found out today when the local M-L rep came over (and you should be able to get the same level of service from your dealer), is that I had a couple of little bumps in my bass response with the Summits, even though my room is pretty well treated.
We minimized this by using the level controls on the Summits to roll off a little before the bump, then place and adjust the sub accordingly. I just set everything up by ear to get it playing, but once we set the meter up it was pretty easy to dial the bumps out.
You will need a radio shack sound level meter and a set of test tones going from 20hz to about 120 hz, in 5 hz increments if you can get it....
The bigger sub will move more air, and you can probably just back off the level. Any chance your dealer will offer an in-home trial?
Might even consider a pair of the dynamos. Depending on how your room is, you might get more even bass with two small woofers! I believe a pair of Dynamos at retail is only about 1200 bucks....
