Dave, I'll add to my reply to your question in the other thread(by the way, that makes it harder to come up with a coherent response)by pointing out that bigger speakers(including Axioms)quite often are more sensitive than smaller ones and so require less, not more, power.
The 6 1/2" M60 drivers aren't "mid/bass", they're entirely bass. The mid-range is handled by the 5 1/4" driver and matches well with the same driver in the M2.
If the back surround speakers in a 7.1 setup are 16' behind the listening position, they should be separated by about that same distance if it's otherwise convenient.
Good reminder on bigger speakers being more efficient, hence louder at a given volume. Given three subs, my fronts and all surrounds being rightly set to 'small' & 80Hz x-over by Audyssey, I see your point on the M60' drivers: dual 6 1/2" 80-200Hz, 5 1/4" 200-2000Hz. M2's: 5 1/4's doing the full 80-2,700Hz, which I guess more importantly closely matches the QS8's 5 1/4's doing 80-2,500Hz. Is it not significant (to timber matching) fronts to surrounds that the 5 1/4" surrounds are not focused on a small freq. range as the front 5 1/4's?
16' separation on backs:
I can do this, but it will put the left rear sandwiched between the top of kitch. cabinets and ceiling (≈ 2" avg above spkr with angled ceiling) and butted against a vent enclosure on one side, with the small space on the other side where ceiling slopes to the side wall about 2' away. Bottom line, this speaker's front grill will not have surfaces to reflect off of except the close sloped ceiling. will th efacct that the 'cabinet' of the speaker is directly next to surfaces on it's bottom and one side?
The rt. back can go anywhere along that sides wall. The next inward opportunity for the lft back is moving ≈2' towards center (on the room center side of the vent enclosure on top of cabinets).
