In reply to:

Had the additional pleasure of hearing the Sennheiser tube headphone amp running thru HD-600s. Incredible. But it had better be for $12K.

Actually... Their $15k USD rig is the Orpheus. It does not use the HD-600s at all (what a rip-off that would be if that were the case, as HD-600's can be found for $400 USD)... It actually uses an 1 micrometer thick electrostatic diaphragm and that is linked to their electronic tube (aka valve) amplifier.