bridgman,
You make some interesting notes here. Let's compare.
1) Yes indeed, setting SPL to match is an absolute necessity. I also agree that an assistant is a necessity and the wife makes an excellent assistant. Just don't call her that.
2) The M60s were 'boomy' closer to the wall. Not a surprise really, but what i found interesting was the distance you eventually settled upon. My M60s presently sit b/w 12 and 14" out from the back wall. Amazing coincidence i'm sure.
3) Your flip floppy report of how the M60 sounds vs. the M2 is nearly identical to what i heard with the M22 vs. the M60. I know i know, you are shaking your head..."don't talk me into buying the M22s" you're thinking. The M22 literally sounded like the small version of the M60, a little brother so to speak. Their character sound was virtually identical except that the M60 had more bass extension, more bass presence (all this is not unexpected) and overall just a LARGER sound. Again this should not be unexpected as we are comparing a bookshelf cabinet to a full size floorstanding model. Larger cabinet (and drivers) equals larger sound. In our listening room, the M22s also ran out of gas whereas the M60 filled the room nicely with sound.
If i recall, spiffnme also had the chance to hear the M22 and M60 and reported a similar opinion on the differences between the two.
What i really would love to audition is the M40 or M50 against the M60 (alternatively the M3 vs. the M2).
