I own the earlier version of the M40. It is most definitely tuned to be quite flat in response compared to the M50's slight V-shape sound signature. And by flat, I mean a real flat EQ. Very nice tool if you need this kind of response for your applications. The M40 is a favorite of my father since he uses it primarily with his guitar.
The M40 will need EQ if you want to achieve extra bass with the headphone.
I can't really put a figure on what pieces dictates price...but in the process of stepping up from the M40 and M50, a few things are highlighted:
-You go from a 40mm driver to 45mm driver. The larger driver can easily hit the lower frequencies with a bit more more pressure as the driver can displace a bit more air. As such, the M50 can actually output low frequency effects with significant presence as you can feel the sound pressure going into your ear...similar in effect of a 10" Subwoofer sweeping through 38Hz-50hz test tones while shattering windows LOL But it's because of this effect that a lot of drummers out there prefer the M50 model.
- Wider Frequency range but more importantly you go from 98dB to 99dB sensitivity. The M50 are more sensitive and will require less power to achieve the same listening volume.
All in all, try auditioning them if you're able to.