Mullard CV4004

I purchased some CV4004s and found the same results as Zinfan2. However, just recently purchased a matched quad of Mullard ECC83 tubes from 1959 and these sound fantastic. Sometimes it all depends on who you buy them from. The vendor in CA I bought mine from and the most likely vendor you purchased your Prima Luna from sold me the set of Mullards I did not like (very microphonic and he wouldn't take them back). The Mullards are hard tubes to come by in excellent matched condition. More reasonable are the RCA 12AX7s from the '50s. Andy at VTS carries a good stock of them and can match them nicely to your amp.

If interested email me as I have some '62 Telefunkens that could be available.

If you really want the best 12AX7,
try the Amperex 12AX7, 7025 and the old Tesla ECC803S ( Telefunken copycat ) would be better choice.

...and don't forget the NP GrooveTube 12AX7M, an intended duplicate of a Mullard ECC83. They sure sound good in the frontend of my poweramps. Probably they'll be less expensive than the old ones recommended above.
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I'l be the only one to argue that it was an improvement over the stock 12AX7s I got with my Jolida CD100. I have several variants of these tubes in NOS Mullard which do in fact sound better to my ears (I have used them in other applications,) but for the price they made a tremendous improvement in the sound of my Jolida Tube output CD. I put them in and my wife won't let me roll them. She is a violinist who finds the tone and timbre excellent. I haven't been able due to anti-WAF to try them elsewhere they have been in place for over 2 years. Maybe they are just an excellent pair, bought them from a friend who got them from the Prologue dealer- I think about 4 years ago. So in sum, I think they are an improvement. How much I can't be sure because I secretly rolled all the other tubes in (her) my system so she wouldn't interfere, now its sound quicker and more transparent overall. At the same time I switched the ICs to Silver without her knowing etc. So take that for what its worth, they may not be the ultimate best choice, but if the price is right, I would give them a try. They do take some time to break in.

I found the 4004 to have a big and warm sound