Bi-amping does not mess with the resistance at all, and both amps in question can handle much more difficult loads than the 8ohm RTi10's anyway. Bi-amping with the 7125 will probably be almost indistinguishable from running 2200's for your mains, and I don't know how much benefit the 2200's would give you for the center and surrounds. Either upgrade will be a nice move up from a receiver's internal amps.