Contact Kevin at Upscale Audio ( www.upscaleaudio.com ). Kevin carries the Unison line and is an expert on recommending upgrade tubes to fine tune the sound to your taste.
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Here's an alternative view ... avoid the cv4003 tubes, or any advice from kevin at upscale audio. He's earned himself a horrible reputation with a lot of tube-o-files for selling crap and misleading people in order to unload inventory.
Example: the cv4003 tubes, which he heartily recommends (and charges more than his competitors) are 1980s vintage and not even made by mullard. They are made by various companies that bought mullard's tooling and trademarks, and they sound shrill--typically better than contemporary tubes, but nothing like vintage mullards.
If you want excellent advice and better prices, i'd recommend the guy at Vintage Tube Services. He gave me the straight dope, and when I told him I couldn't afford the vintage mullards he recommended some vintage RCA black plate tubes. He sells some that are used (but that still measure as new) for a serious discount.
Anyway, these vintage RCAs sound head and shoulders better than the cv4003s that i used. In general the cv4003s sounded a bit better than the stock tubes .. a little less hard .. but they recessed the midrange and took a lot of meat out of the vocals. The RCAs as smooth and warm, no complaints. If you can afford the vintage mullards, these are supposedly even better by quite a bit.
I have had (and continue to own) a Unison Unico which I purchased on Audiogone in 2010. I have tried a number of 12au7 tubes from Upscale and other sources. The ones I have tried:
1. Radiotechnique 12au7 - came with the unit and does not sound good at all. Not sure of the condition of the tubes when I got them, but were only used for a few days and now they gather dust.
2. RCA Cleartop 12au7
3. Mullard CV 4003
4. Seimens E82CC/12au7
Among these, the Seimens is the clear winner - beautiful midrange, a great low end, clear, airy highs, and fantastic dynamics!. From low to high they are the best. I like these so much I bought a 2nd pair as backup.
I got the Mullards after reading their praises and thinking - 'if these are better than the Seimens this will be amazing'. The Mullards have a very nice midrange as well - comparable with the Seimans, but the upper end in comparison seems a bit soft and not as extended, while the bottom end seems to be missing some extension and 'fullness,' for lack of a better term.
At the same time I got a pair of the RCA cleartop. This is a nice tube, but definitely a step down from the Seimens. Slightly better upper end than the Mullards but not as good in the midrange; bottom ends comparable.
To my ears, Seimens-Unico is a match made in heaven!
Enjoy your new amp - this is an underappreciated gem.