Questions from a Tube Preamp newbie

@stereo5   Wow.  I'd have to check receipts but when I bought the CV 4003 about 15 or so years ago they were about $100.00 for a matched pair.  That was back in my early days with my tube line stage and just went with the advice I was given.  Perhaps I might have obtained better results with the Mullard 12AU7.

I never leave tube amps on when I'm not around as I've had small signal (preamp) tube commit "tubicide"…once in a guitar amp I left on after a rehearsal (for a week) and that amp never really recovered (sad), and another time a well regarded new-ish NOS Phillips AT7 killed itself right on front of me during a listening session…jumped up and shut down the amp before any damage happened. If tubes have a break-in period I'm not aware of that as they seem to display whatever their tonal characteristics are pretty much right off the bat, although the rest of the amp might need warm up. I noticed this when swapping out tubes having just shut an amp down to let it cool a bit…many many hours can be gleaned from a hard driven signal tube, proven by guitar players everywhere when you'd just forget about 'em until the amp starts to get weird…years...

I have a lot of the CV4003s from Upscale.  They used to be $35 each for the Platinum Grade.  These are the ones that I've had no problems with.  I've used them in DACs, preamps, phono stages, all over.

I also have several other types of Mullard 12AU7/equiv.

I'm a tube newbie like you and it looks like you're getting great recommendations specific to your preamp. When I got the tube rolling itch, I contacted Aric at Aric Audio directly and he recommended a specific set of tube brands and models; rectifier, voltage regulators, and 12ax7 signals, and I swapped them one at a time over a month period just to gauge the improvement. Collectively, they took the preamp to an entirely new/better level. His recommendation was based on his own personal experience rolling many different brands/models in the same unit he uses regularly in his own system. So sometimes asking the designer may be a good approach. Mine came with JJ 12ax7 signal tubes and apparently the Electro Harmonix Gold pins work best in this particular model. Total cost was $85 for 5 tubes. By the way, in my case, the overall difference after the swap was not subtle or just a matter of taste, etc. It just made the music sound better. I asked him why doesn't he sell them with these tubes and his response was I was the first person who contacted him for tube suggestions and secondly most people have a favorite brand/model in mind and just go ahead and roll tubes regardless of what comes with the unit so why put more expensive tubes in. By the way, he also gave me some recommendations for upgrading a small number of the capacitors to Audyn Copper to make it sound "crazy" good. That's my next plan.
Good luck and enjoy the music.