In re: 5.1 vs. 7.1, let me throw my hat in the ring ...
7.1 is cool. But it's not the mind blowing experience (compared to 5.1 with a pair of QS8s) that 5.1 was (compared to a 2 channel stereo set up).
It's just that the QS8s are so damned good at spreading out sound that there are no gaps in the rear sound field that really REQUIRE 2 more speakers to fill in the space.
But, then again, 7.1 is fun for bragging rights. Also, you probably already bought that 7.1 receiver, and if you're like me, you want to squeeze every last sound out of it. For that reason alone, 7.1 is justifiable.
Also, your room isn't that big. Try a 7.1 setup before you go with the dual subs. You'll get plenty of deep bass with a single sub for now. Later, when the funds get replenished, you can splurge on that second sub if you think you'll need it.
My suggestion would be:
2 M60
VP150
2QS8
2M2 (rears)
1 EP500
Total price: $3400
You might also consider going with a dual Hsu Research sub setup, for about the same price as a single EP500.
