Hi USG! Good to see you back in the thread!
A couple of comments on the above I think you may find intriguing - first, as a fellow (now former) woo 3 owner, I sincerely doubt the validity their posted specs on their Otl's. The Woo 3 and 2 are both much slower and tubier sounding than the crack, which is faster, more detailed and more dynamic by comparison, especially with the ccs boards in place. Crack is objectively a better more sophisticated circuit. The cost differential is labor and fancy case work.
That said, in his diy comparison thread, wyldrage measured (with a humble emu 0404 setup) the crack as having distortion higher than what schiit publishes for the vali - around .8% if memory serves. But it really does sound better, and cleaner, than the woo 3 and much better, to my ears, than my o2 with high impedance Senns. My guess is that woo's measurements are off by a lot. Jack Woo personally commented that his transformer coupled amps sound more solid state than the OTLs. Woo's published distortion measurements (way worse for the transformer coupled than for the OTLs) can't be reconciled with that comment.
Finally, I'm actually a fan of measurement first amps, as well as good tube amps. I am presently working on both a wire se se (half finished) and the really cool agdr booster board for my o2 (debugging), which brings a battery powered o2 within spitting distance of the wire. Even stock, I love my o2 with my alpha dogs, my westone 4s and even my dt880's, but for whatever reason, it just doesn't work for me at all with my hd650's. The engaging magic just isn't there to my ears. More power to those who find complete happiness with what is irrefutably an incredible budget performer, but notwithstanding their measurements, nonfeedback SET designs seem to just play really nicely with hd650's as I and many others hear them. Maybe the distortion is in a sense simply compensating for the room effects we don't get with headphones?
Keeping it real though, I was advocating the crack in this thread back in 2011 and was largely ignored. Now it's all the rage and I'm still a big fan of my crack and listen to it with my hd650's just about every day - but even hotrodded, it isn't the equal of a zana and certainly not the equal of bottlehead's mainline or others of that caliber. A great match for hd650's, no doubt. Better than even the woo 2 at twice the cost, I would say yes. But it is hyperbole to suggest that it is where the buck stops for this or any other headphone in my opinion.
Edited by skeptic - 4/5/14 at 11:01pm