How many rows of seats will you have? I have a room that is 24 feet long and a hair wider than your room and if I have a single row of seats, I would have stayed with 5.1
I have 7.1 for the space, but really only because of the two rows of seats. I really don't see the benefit of 7.1 for most cases.
With that, use the money that you save from the second set of QS4s and upgrade to the above mentioned VP160 (absolute must in my book since the center channel is arguably the most important speaker in a home theater), get a pair of QS8s which are a nice match for the M60s and VP160 and go with the EP500.
You will have a pretty sweet 5.1 setup vs. a good but somewhat lacking 7.1 setup. Your 5.1 setup will have a much better timbre match in speakers and a more appropriately sized subwoofer. With the 7.1 setup, the components were more along the lines of for a small space (minus the M60s).
You can always wire for 7.1 and add on later if you absolutely feel the need, but at least you won't ever feel the need to upgrade the existing 5.1 speakers you already have.
