I'm far too lazy to check, but I believe the 2600 and 2700 also have the ability to bi-amp the fronts with 2 of the "extra" channels, while the 2500 doesn't. You should check into it since that's definitely an advancement that could help your situation.
I'm about to buy a 2500 myself, but there's really nothing in the 2600 that I absolutely need (want, yes...need, no), and I'm not worried about the power issue which is why I don't know right off the top of my head.