For what it's worth, I'm keeping my M50 and getting rid of my SRH750DJ. I just think the M50 have a great sound, and really works with all kinds of genres. I found the 750DJ to be a bit more like a M-Audio Q40 sound, but with too prominent of bass that veiled a lot of the sound. The highs will pierce through the veil every now and then, but the headphone overall just felt out of place. I know some people will really enjoy it as it is quite different from other similar bassy headphones, but it wasn't my cup of tea. I would compare the 750DJ to Equation RP21 and M-Audio Q40.
The M50 just seem to fit in a different crowd I felt, and seem to be more enjoyable for vocals than any other bassy headphone I've listened to. I actually found the M50 easier to drive with my Sansa Fuze than the 750DJ. Both, however, are easily cable of being unamped.