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.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated