MLP = Main Listening Position, yes
Sub recommendations will depend on budget
In terms of crossovers, definitely stick with 80Hz for the RC-10's. The RC-30's / RC-LCR can take a 60Hz crossover but there is really no downside to 80Hz, and more of the bass will be shifted to the subwoofers.... but with your current "inferior" subwoofer, you might find it actually sounds better with the front 3 speakers at 60Hz since they probably produce cleaner bass.
Remember also that you can set a separate crossover for multichannel vs. 2-ch stereo on the Denon AVR's (Manual Setup > Audio Setup > 2CH Direct/Stereo). So you could go 80Hz all around for movies, but switch to 60Hz for critical 2-ch music (for example).
Awesome thanks I will do that when I get home for 2-ch and leave it as is for the rest. I dont listen to music much a bunch...my ratio is about 80/20(HT/music)