Red, congrats & welcome!
I have a Yamaha 5660 and the "phantom" centre works awesome. I tried an old junk speaker I had kicking around as a centre, and I just couldn't stand it. Threw it out and went back to just the mains.
Not sure about sending the LFE to the M50's, though. I would probably skip that all together. The risk of damaging something is far too high. Be patient grasshopper. A sub is in your future. (And that's what should be next on your list.)
As far as the YPAO, you will get mixed advise on this. If you do a search, you will find plenty of threads debating the pro's & con's. I personally, am in the "don't use it" camp. Try without it for at least a week or two. Tweak the system by moving your speakers in & out, left to right, etc. Then for kicks, run YPAO. If you like it better, who's right?
