There will be a timber mismatch using the M50's, not that it is critical for the surrounds but for multichannel music it is preffered to have a timber match on all speakers. I would suggest M22's or M60/M80 for direct radiators, but I would recommend the QS8's over the 4's for a perfect timber match and the QS8s over a direct radiator for surround duty.
The QS series is very forgiving on placement do you have a picture/drawing of your room, maybe we could suggest some places for the QS series to go.
