I have the same setup as you and I use the M3's as surrounds but placed at the opposite end of the room from my M60's.Like you I have no place to put the QS speakers.It works great for me,I listen to music mostly,but movies sound great.My room is about 22 by 14.I wanted more of a full range surround and with the M3's sometimes I play my system in 4ch direct or just turn off the 60's and listen to the M3's by themselves for variety.

M60-VP150-M3-EP350
Boston AVR7120