I had an interesting experience last week as the bug to upgrade my HD600s has started biting harder and harder. I sat down with HD600s, HD650s, and HD800s (all well-broken-in) -- along with Trio Wanderer's recording of Haydn's piano trios nos. 39, 43-45. (I wanted to limit myself during this session to one recording that is both excellent and very familiar.) File was lossless from CD played by iPhone through AudioQuest DragonFly Red (which is my portable setup). I dismissed the HD650s immediately as sounding veiled, congested, and foggy on this recording. (This is how I remember them from listening years ago.) Overall, the HD800s were exquisite -- with so much air and delicacy. Strings were gorgeous. There was something nagging about the HD800's portrayal of piano, however. Timbre was off. I don't know quite how to elaborate on this ... it was just inaccurate. The HD600's representation of piano was qualitatively better -- and still the best I've ever heard. Spot on. I was surprised. I've been gearing up for a rather significant purchase -- which I now question as being necessary. Whatever the case, I've slowed myself down, joined this site(!), and expanded my options to include remaining with the HD600s. I (still) think that they're wonderful ... and get many things absolutely correct. I'm going to continue these listening tests -- much more to be done! -- and include other 'phones as the opportunity presents. But, I'm wondering if the HD600s can be improved upon (for me) with any reasonable amount of money.