One of the hd650s did sound good even with my old FiiO E9 (not even the K version yet). The two other pairs sounded worse. One was the version with the old black felt material covering the driver, and the other was a silver driver material that didn't sound quite right.
I actually like the senn 5xx series and don't find them muddy with the right amp (low output impedance). But as for the ma900 vs hd650 comparisons on this thread it makes me curious as to why no one has really said that the sony's are veiled or anything like that when the hd650s are at many times veiled with the wrong equipment pairing. Heck lots of older hd650s are veiled regardless of what you pair them with.
Just curious by the way if anyone here has owned both the ma900s and the hd600s and can say whether the ma900s have things that it can do better than the senns.
EDIT: I just saw Astroid's post. Anyone else with hd600s to add a second opinion?