To give some perspective, I spent a good 3 hours demoing various combinations of M22, M60 and M80 at volumes between 70 and 90db (things got louder as the session went on) without experiencing any fatigue. The old Axiom demo room, though not specifically treated, was not particularly lively.
I went with the M80s. My apartment is concrete construction and I have a lot of bare wall/cieling/floor to generate reflections. On a very few recordings, such as pipe organ, I find the high end and some bass notes a bit too much. You can feel the energy from reflections build on notes that sustain for any amount of time. It has everything to do with the boost from all those reflected sounds. I am quite confident that once I treat some of the reflecting surfaces to absorb and diffuse reflections, I will get back to the clean sound I heard at Axiom.
For the most part though, even in a highly reflective room I do not find the M80s (which have the same dual tweeters) to be fatiguing
