If they are truly spare speakers then give 7.1 a try, no need to get fancy initially, just set them on a ladder or similar behind the listening position, if you like the added effects then start planning the wire situation out. If you don't like it then remove it all and use the M2's as PC speakers or whatever, It eill still come down to your own preference. FWIW, I ran 6.1 for a number of years and my corner loaded situation the 6th channel was a great improvement; I had a HUGE gap between the L and R surround channels.
