Just to throw my own 2-cents into the mix.
I own the M50s (a very satisfied listener) and have auditioned the SRH840's a couple times at the local Guitar Center. I know that's not the same as owning a pair, but it's better than nothing--I actually listened to the Shure's earlier today when I was comparing them against some Beyers.
In any case, the M50's are excellent all-around cans IMHO, and not limited to hip-hop fans or bassheads. There's been so much use of "recessed mids" to describe them, both here and in almost every other thread about the M50's. I think until you've heard neutral or even forward mids, you won't really understand how much of an exaggeration 'recessed' is. I will agree that the SRH840's sounded much more forward, the mids being very much apparent, but I think unless you're actively looking to hear out the mids, you won't be missing anything with the M50's.Granted, that's just another opinion, but if that's what you want to base your decision on, I highly suggest searching some of the other threads for more input on this same type of debate (i.e. SRH840 vs M50).
Some other things to consider that may/may not help your decision: the M50's are much more portable, being able to fold up should you ever want to carry them somewhere. The 840's are generally a little more uncomfortable for long-term use, but if you have sensitive ears, then the M50's may feel more uncomfortable because of the tighter headband (this can be stretched out though, and actually 840 pads can be used on the M50 to increase comfort). The 840's also have a detachable cord.
