Had some friends over last night, ended up listening to the M60s for a while. All liked the sound but I had to agree the imaging wasn't all it could be. I suspected the problem was reflections of the (very bare) rear wall so tried the obvious solution... grab a couple of small pillows and hold one behind each ear to block out the effect of the rear wall reflections.
You can guess your friends age when one of them looks at you holding a triangular pillow behind each ear and says "you look like Peter Gabriel". You can guess your own age when you know what they are talking about (the batwing headpiece Gabriel wore when performing Watcher of the Skies back in the early Genesis days).
But back to the sound... the difference in imaging quality was nothing short of incredible. With any kind of heavy damping material behind my head the imaging went from so-so to fantastic, better than I had heard with any system to date.
Haven't figured out what to do about it yet (I'll need something larger than a pillow if I want to mount it on the wall rather than the back of my head, thinking about a big wall hanging I saw of a moose drinking from the edge of a Northern Ontario lake.
Anyways, if you have a reflective back wall and you aren't happy with your imaging, shoo everyone out of the house and hold a pillow up behind your head centered on ear level. If it makes a difference... you know what to do.
Note that this won't do anything for reflective side walls... I think you have to treat them at the wall not your head
... but if the back wall is the problem this will tell you very quickly.
Image of Peter Gabriel linked from "The Musical Box"s web site at www.themusicalbox.net.