Thanks for the detailed reply. Difficult to comment further - especially as there are a number of factors which you may have which are impossible for me to replicate ......
- By the sounds of things, the HD600 you tried were 'aged' and not sure what condition pads were in - this can dramatically affect sound
- Not sure how old HD650 were that you tried, New silver driver HD650's are much closer to HD600 sound
- My critical comparisons were in a quiet room, volume matched (SPL meter), on consistent equipment with same tracks. Yours were somewhat different
In my listening, the HD650 actually rendered instruments with a deeper timbre slightly better (cello, piano etc), Anyway - I wish you luck on your search. If it's any use, Rizki compared same HD600 and HD650 on his own equipment and reported similar findings to mine. Others recently in this thread have said same.
Either one you choose should make you happy - both
wonderful headphones and worthy of hifi status.