I stopped playing around with bypassing the Dayton film caps in the Crack, and rolled in a 'platinum' NOS Mullard CV4003 from Upscale. More prominent (and fuller) mids, more air around instruments for sure, deeper bass, but there was still something missing.
Got a vintage RCA 6080 from Jim McShane, replaced the stock Sylvania 6080 with the RCA tube and, Whoa, Nelly!! I'm just about not believing what I'm hearing thru the HD600s with this combination.
Why did it take me so long to discover this? I wasted several thousand dollars looking for what I could have had for closer to $750.
A Crack and a pair of high impedance 'phones ought to be mandatory continuing auditory education before folks go spending big bucks on audio gear chasing 'the absolute sound'.
"Too soon old;
Too late schmart"
The late Mr. Fox, Fox's Deli, Rochester, NY
Mike P.