OK, I've spent a few more weeks now with both speakers side by side and if I had a gun placed to my head I'd have to say the M3s are marginally easier to relax with. The Mon 11s are certainly not a fatiguing speaker by any stretch, but I have on two occasions noticed a slight headache after a 4-5 hr session [strictly listening, no reading or anything (other than, perhaps, liner notes)]. This has not happened with the M3s, to date. Mind you I have had more marathon sessions with the 11s than the 3s, and only twice with any negative consequences. The session variables are legion, so I'm not sure what can be read from the experiences other than that neither would be problematic.
All in all I can't see the M3 being even remotely fatiguing. Kinda sorry now that I didn't bid more aggressively for the last M40.
