[MixMasterMan]

  Now, I'm awakening this thread...
What about their comparison for metal music (mostly Megadeth, Dream theater kinda bands, thrash-progressive metal)
But at the same time, they should not be lame overall. I mean, I need great sounding high clarity cans for metal music and decent, adequate overall.

 
The M50x & 840 are pretty similar sound wise. Either one will do, but the M50x def more comfortable, has a punchier (some will say bloated or hyped) bass, and a slight treble shelf at around 2khz (which will bring out details but may be a little harsh depending on how sensitive you are to that range). It also sounds a little wider than the 840. The 840 is smoother and more flat in the mids, while the M50x is a little bumpy, and generally more recessed. For Metal, I'd actually look at the M40x over either. It's flatter than the M50x, and the more even treble makes busy metal more clear and even across the board. The M50x is the loudest and easiest to drive of the 3, and the most comfortable. But the M40x is nice too. Just has more clamp force. - I'd recommend the 840 more if they were more comfortable, but they are heavy and can slide off your head if you move around too much. So, I'd say try the M40x/50x, and pick which one you like best!