Hello scottilad,
I'm sorry to hear you find the M50s "boomy"; they shouldn't be, but I suspect it's a combination of the standing waves in your room, which cause areas of excessive bass reinforcement (and areas where you'll hear deficient or non-existent bass). Think of your room as "the forgotten component" before you rush out to try new electronics.
The closer speakers are placed to room surfaces--walls, corners, etc.--the greater the bass reinforcement. Try moving them out from any walls, and also try shifting where you sit by a foot or two. Usually some combination of moving the speakers and/or your listening chair will yield a reasonable compromise between fairly smooth, non-boomy bass, and room decor.
Are you certain the bass boost/"loudness" button on the Optimus is off?
In terms of imaging, how far away do you sit? With tower speakers, I find you have to be at least 12 feet back to get the imaging to really lock in properly.
I'll answer some of your other questions later re NAD and its power ratings.
Regards,
