I have not listened to the M80v4 vs. M100v4. Only M80v2 vs. M100v4 (and all the other Axioms you see in my sig).
The soundstage and imaging will be very similar. Soundstage and imaging is very similar across all v4 Axioms. The difference between the less expensive and more expensive models is the degree of resolution in the imaging, expanse of soundstage, ability to reach lower and dynamic capability.
I would expect, judging from the frequency response curves, the M100v4 is more linear in the bass than the M80v4. Also note the M100s have more dynamic capability than the M80s. The M80s are linear to 1600W peaks while the M100s are linear to 2400W peaks.
The M100s may sound a bit better in the lower highs. They are crossed over at 2KHz whereas the M80s are crossed over at 1.8KHz. The lower cross-over on the M80s may push the tweeter closer to its self-resonance. Andrew worked to lower the resonance on the v4 tweeter but still - 200Hz closer to the resonance is 200Hz closer than the M100. BTW, this higher resonance on the M80v2 is responsible for the shrillness that you may be able to hear. I know I sure could. Thankfully the tweeters on all models that I've heard is sublime.
