Welcome mdavis,
I have to agree with St_PatGuy, I do not think upgrading to the QS8's in your situation will provide a very noticable improvement in your system. If you were moving your system to a larger room, the QS8's would be a good upgrade because they can play louder and lower and fill a room much easier without strain. I use the QS4's in a moderately sized room and they work wonderfully! I love them.
I think a better upgrade in your case is to get a matching centre channel (Axiom VP150) which will improve the timbre matching with your M60's. This can help provide a more seamless front soundstage for your system and could provide a significant improvement.
Good luck,
