Hay, are you saying the M3's are not a good match for rear surrounds with the M60's and QS8's? Does it matter that they would be 15 feet behind the listening area?
For H/T they were fine, mine were about 4 feet behind me. If there was a pan around the room, the sound signature was noticeably different.
For multi channel music I didn't care for it, so I changed the wiring so the M3's were my Speaker B and I spent a lot of time sitting in the middle of the room going back and forth between them and my M60's. I listened with the M60's up front, then turned around and switched to the M3 in back. Once I realized I wasn't 100% happy with the 4 surrounds, plus the M3's were large, I took the M3's down. Once they were down, I played with using them up front as Presence surround speakers. This was neat but meant I could not run 4 rears and I would have preferred smaller M2's for the presence speakers.
Finally, I put the M3's on top of the M60's and switched back and forth A/B in 2 channel listening to a range of music. To me the 2 sounded very different but both excellent. Each one had their own pros and cons and I loved them both. I had a couple people over one day and turned on the system (2 channel music) with them not knowing which speakers were playing. I put on a few different tracks and they mentioned that my towers were killer. I let them know that they weren't on, it was the M3 bookshelves on top.
To me the M3's are just a little more laid back than the M60's. The detail is still there for the highs with the M3's but they just seem silky smooth. Switching quickly from M60 to M3 I prefer the M60. But, if I leave the M3 on for more than a couple seconds I find myself sitting there listening to a few songs.
For your application with the M3's being much further back your findings will most likely be different. I would possibly look at the M2's as an alternative, that or more QS8's as I find them great with music as well. My room really doesn't warrant a 7.1 system, but when I was having it built I knew it was the coming standard. Better to run wires when planning and when I sell the house I'm sure the next person will be glad that I did as well.
Good luck and half the fun is going through the set up and tests to find what you like best.
