Separate power amps typically perform better than amp sections included in receivers, even when dealing with good quality receivers. This becomes even more true when venturing into the realm of the more difficult loads presented by 4 ohm speakers. Adding some sort of external amp should definitely benefit your system.

The speakers in your system all have fairly typical efficiencies (90dB to 91dB). My speakers have similar efficiencies, and the 200W available to my mains (using M200's) and 165W available to my center and surrounds (using a 750) works very nicely. The RMB-1075 shouldn't have any trouble driving your speakers comfortably. The one system I've heard with an RMB-1075 in it had a somewhat harsher sound than I like, but from some other comments I've read I think that harshness was due to the RSP-1068 that was upstream of the 1075. (And of course there's also the matter of personal preference...) The Model 755 would give you some extra headroom, which might come in handy later if you changed speakers and of course wouldn't hurt anything in the interim. Both are good amps.