I loved D18 with my WA22 and HD 650. However, I specifically chose tubes and cabling that sped up the HD 650, reigned in and tightened up the bass, and brought out the treble a tad. Once each of those three areas are influenced, I find HD 650 scale fantastically. I have heard almost all of the flagships on my rig and, while I can appreciate their technical superiority, HD 650 scale so high as to be totally satisfying for me.
Then again, I have heard HD 650 sound like they have completely muddy and boomy bass, slow and unable to keep pace, and veiled where symbols and piano sound totally off. Synergy can really play a huge role in head-fi from my observations.
Originally Posted by project86
I suppose it would come down to the amp, and maybe even the tubes in that amp. And of course listener preference - some would say the HD650 sound too warm/dark, even driven from a super clean, neutral amp.