You will be able to tell the difference between the 840 & the M50. You may like the sound signature of one or the other--regardless you will hear how they are different.
Don't forget the very accurate KRK KNS-8400, which goes for about the same price as the M50 in recent weeks.
there is no way for me to try them out, i'm just going to order one or the other. i'll look into shure more, people say it's a bit heavy on the head due to it's material i guess.
as you said, the KRK KNS-8400 seem on the spot accurate, i think i want the bass to hit a little harder than normal; but agh the mids, the recessed vocals and stuff with the M50's, that concerns me. this is difficult (: