Is there any chance the Anthem amp has slightly higher gain than the amp in your receiver and part of the difference in sound between them is that the levels are just a bit higher going through the Anthem ?
I wouldn't expect a noticeable difference in sound between the two amps when running surround content through them at normal levels, although (a) if the levels aren't exactly matched then all bets are off, and (b) if you are listening only to the surround channels and cranking the levels up you may be playing loudly enough to be crowding the limits on the receiver's amp... but I don't think you would hear the same difference when playing normal content.
I guess I'm arguing for "use the Anthem front & center, use the receiver's amp for surrounds, make sure the levels are right, and enjoy".
But 11.1 sounds interesting too
