This is a pretty limited comparison. I relied on the reviews of others before deciding to try a Mullard/Holland GZ34 in place of my stock Sovtek 5U4G in my stock Woo Audio WA6. (I should have noted the name of the Head-Fi.org member that compared about eight rectifiers and came up with the GZ34. Buddy, whoever you are, I owe you a beer.
Well, I'm amazed that an amp that I loved so much could be improved so much. The new cryro'd GZ34 set me back $250, but it's worth every penny. The Sovtek is a sexy, voluptuous looking tube, with large size and nice curves, BUT the compact little GZ34 has a much richer, complex sonic presentation. Bass has much more slam and the mids are rich with with overtones. The highs are crystalline, with no hard edge. So, I'll use the Sovtek for burning in cans and as a spare.
I used my AKG K701s and Ultimate Ears Triple.fi 10 Pros for the comparison. I tried raising the volume with the Sovtek to see if it just needed more gain, but that was not the case, it still sound plainer and less rich. I listened carefully to the highs and the GZ had smoother highes, with more harmonic content. I used Chesky's "The World's Greatest Audiophile Vocal Recordings" on SACD for the comparison. It's actually a very good SACD with lots of bass, a variety of excellent vocalists and many instruments in the arrangements, from solo bass to full orchestra. Of course, being Chesky, it's well recorded.
I bought the GZ34 off Audiogon. I'd looked a few weeks back and only saw one or two Hollands at prices over $600. That made the $250 seem a little more palatable. Apparently Mullard bought up lots of European tubes, relabelled them and cryrogenically treated some of them. I suppose the non-Mullard Hollands are even harder to find, perhaps explaining the $600+ price I first found. I can't imagine such a tube being more than twice as good as this one.
The seller may have one or two left. He goes by bprager on Audiogon if you're interested. Hmm, maybe I should buy a spare...