I myself found the k612 just meh. The tonality isn't as offensive as the old k701 I had but that cheap sound is still there. A fake timbre glazing over everything. Bass/mids/treble balance is okay as a whole but I just can't get past that hollow sound. I would take a sennheiser hd555 over the k612 or k701 for that matter any day of the week.
I haven't heard the ma900 yet but if it is closer in tonality to sennheisers plus add the open angled sound, I may prefer them more. Heck, I need to buy one already :D
I personally found the HD 555 grainy, artificial sounding, and lacking in definition, I did like it's soundstage, imaging, and bass response though. It could be a case of bad system synergy on them though, I do still have the HD 555 so I can experiment with amplification with them sometime. Did you try it on any other systems? The K612 is anything but hollow and unnatural sounding on my system, it's full and robust sounding without even a hint of hollowness, it could be bad system synergy you are experiencing.
Edited by kman1211 - 11/23/13 at 5:19pm