Mullard CV4004

...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

Pure garbage! These are modern [mid-1980's] tubes, not NOS tubes. White boxes and no Mullard name on the tube. Stay away!

I use Mullard CV4004s in the phono section of my VTL tl5.5 and like them just fine. They are a big improvement from the Sovtek 12ax7s that were in there when I got it. The Mullards are quiet and have a nice full mid range and a smooth top end. I used to use them in the line stage but I found I like early '60s Tung Sol (American) grey plates 12ax7s better. My preference in general as been for early 60s American tubes to the "CV" Mullards and 60s Telefunkens I have tried.