As a professional musician and a life long H-Fi and headphone enthusiast I have to respond to this.
There's no doubt that the expectations and tastes of Hi-Fi and headphone enthusiasts differ (at times not inconsiderably) from those of professional musicians and recording engineers. To list these would require much more time than I'm going to spend writing this post, and indeed it could be the subject of an entire thread, but one of the differences, as I perceive it, is that very often enthusiasts place great importance on fidelity (which can mean a whole raft of things to different people) whereas professionals are more interested in a natural balance of all aspects of the production, and especially in tonal accuracy. I have heard quite a few (and own at least one) headphone which is regarded without question as being 'better' than the HD650. Technically, and in very many ways, especially in terms of fidelity, the HD800 is vastly superior to the HD650, and yet very often it doesn't satisfy musically. The reason I believe is that the 650, whilst far from perfect, presents a better balance and most importantly, better tonal accuracy. FWIW the HD650's sound much more like my ATC SL100A monitors than my HD800's do.
There's a great irony in the fact that enthusiasts often spend thousands chasing that last little bit of fidelity that they think will take them closer to the original source, when in fact very often what they are hearing is almost nothing like what the recording engineer and musicians themselves heard.
Individual taste in headphones is just that - individual taste, but to suggest that professional musicians who play the instruments themselves, and in many cases have heard what they sound like on replays in a recording studio, don't know better than non musicians what acoustic instruments should sound like in a good transducer (whether a loudspeaker or headphone) is, frankly, hilarious. Piano is often said to be the hardest test for a transducer. Pianist's generally know what a piano sounds like!
