Receivers are not generally good at 4 ohm loads - few are even rated for 4 ohms. The MMG's are popular speakers, so it's very likely that somebody around here has run them with the 1070 or with the older 1050 (which had comparable power output).

You cannot bi-amp using the 1070's internal amps. The 1070's internal crossovers are designed solely for bass management, and trying to incorporate something into the software that would let folks try to replicate speaker crossover networks for active bi-amping would be a really nasty can of worms to open (way too many variables). The best bet for bi-amping is to do it externally - send the left and right pre-amp outputs to your outboard crossovers and from there to four channels of amplification.