I have to chime in for a sec.
Personally, I find the HD555 mids very tonally flat (as in "sharp" and "flat" like on a musicial instrument, so read flat here as "unnaturally boring"). Neutral? Hmm, nope. I EQ my m50's like so...

...and I get a sound almost exactly the same as the HD555.
Yeah, the treble will sound tishy with the m50, but you'll get that effect with common consumer music on top-level audiophile headphones (i.e. HD650).
Now, I think of the HD555 as a mid-fi headphone disguised as a hi-fi headphone. Notice that peak at 10 KHz. That gives cymbals a nice, slow shimmer, but nothing truly realistic. (That peak creates the HD555's surreal soundstage, though, by the way.) The HD555 response does make most consumer stuff bearable, especially in the mids and treble, but bear in mind that it's coloring up the sound. The frequencies are missing around 2, 4 and 8 KHz and above 11KHz, and that's usually where modern music has its overbearing treble.
Still, who's to argue? This is Head-Fi. Enjoy what your tastes tell you.
Edited by Sonichedgehog360 - 5/19/10 at 10:42pm