I see no problem. Go for it. To be fair, however, they should all have similar input impedances otherwise the output levels intended for the rear channels may differ. You then may have to attenuate either the front or rear channels, if you wanted perfection. Since the rear channels carry predominately ambient information anyway, I'd doubt anyone would notice unless the mismatch was obvious. I would also recommend using the same type of amp (ie SS vs tube designs) so as to keep voicing the same, or similar, but again, I'd doubt anyone would notice, since rear channels should not be "very noticable" anyway, in a properly set up system.