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.
