Another vote for M60 / QS8 instead of M80 / QS4. My belief is that having the extra depth in the surround channels is more meaningful than is the difference between the M60/M80. I have M60's in a very large room and am quite satisfied.
If you bought those two pairs, what does that leave you budget wise? If you could still squeeze in a decent ~$600 subwoofer, I would definitely do that before the center channel. Hsu STF-3, Outlaw LFM-1 or LFM-2 or SVS PB12-ISD are really quite good performers for the money. If you want to have matched Axioms (and get the 5 piece discount), I would consider the EP350 over the EP175; I think the latter would ultimately be a bit underpowered for your room.
I'd be tempted to give the Sony ES a shot, especially with the M60's (which have a slightly more benign load than the M80's).
I ran the M60's without a sub (and with either a phantom or a mismatched center) for quite a while. They are great.
