Hi Randy,
I've never listened to M80s placed on top of subwoofers. The latter setup might lessen the upper bass "punch" or "warmth" even if you have them crossed over to your sub at 40 Hz or 60 Hz or set to "Large" simply because there's less floor reinforcement of the lower octaves given the on-top-of-subs placement.
Soundstage height is a personal preference. If your seating location is farther back in the room, it doesn't matter much, but with closer locations (in my case, about 8 to 13 ft) I prefer the higher soundstage I get from my M22s on stands (which is nicely even with my DLP screen height for video), rather than the lower soundstage from my floor-placed M80s. When you're sitting well back from the M80s on the floor, the soundstage height seems perfectly natural.
Regards,
Alan
