John, I've never heard the M40s but the M50v4 sound like M3s with more linear bass at any level and more linear mids at higher levels. They can go a good 6-9 dB louder than the M3 before audible distortion sets in. I really like them. Maybe you ought to try a pair.
Interesting. The M40 has the "more linear bass" aspect (the bass response is extraordinarily deep and flat) but midrange is probably a bit less clear than the M3's. I haven't directly compared their ability to play loud but my impression is that my older M40's are probably less good in that regard than current M3's.
I'm still trying to clear out enough space and time to finish setting up the LFR's, maybe I can revisit M50s if/when that ever happens.
