The mains and center could easily be left on the floor, I ran my M22s on the floor for a few years, just be sure to install the rubber 'feet' on the bottom to isolate them from the floor. I had run my M22's right on the floor and then added the 'feet' much later and the difference in bass response was noticeable, in a good way \:\)

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70