Replasment for Mullard Long Plates?

There are still a few NOS 12AX7s available. If I were you I would pursue a search. I saw some Sylvanias as a group of 5 NOS NIB for a fair price. There are still Mullards but like you said striaght labeled newish looking ones are almost non existant. I use 5751s for most 12AX7 duties but these too are scarce and not always the best tube for your application. These Noval double triodes do seem to last a long long time, it may be justifiable to invest in a good pair.

RCA Black-plate D getter version from the late 50's era will get you very close to the long-plate Mullard at half the price. Albeit with a touch more warmth.

I second Morningstaraudio; the RCA 12AX7 long black plates are excellent, and would be my first choice as alternative to Mullard long plates. I've used the Mullard long-plates (halo getter), and my recollection is that the RCA are even a touch warmer; it's a lovely warmth, though. I think if I felt the Mullard were clearly superior I'd have stuck with them, but perhaps it's time to give it another listen; I've tended to shy away from their usage due to rarity/expense. The RCA 12AX7, GE 5751 "Silver Clips", and GE 12AY7 "red label" have spent the most listening time in my tube monoblocks (phase inverter position).

I've also tried Mullard short plates, CV4004 box plates, Tele ribbed plates, Tung-Sol long gray plates, various 5751, and a couple other 12AY7. The CV4004 have their good points, but the RCA & Mullard long plate 12AX7 are flat out better. The 5751 black plates are also generally excellent, but offer a different kind of sound.

But what about the Gold Lion 12ax7s? I need more information. At least 17 more words or 3/8s of a paragraph...something...anything.

Wolf -do yourself a favor and check out Audio Asylum Tube Asylum and do a search for the Gold Lion (Russian/re-issue/current production0 12AX7. Or you can just call Jim McShane who sells a lot of the GL tubes about them. He is a very striaght shooter. He has a website as well that you can do an internet search for. My understanding is that the GL 12AX7 is somewhat warm, sounds very good, is well made, and on the expensive side. That last sentence was 22 words is that enough?