I have bought a VAc CLA-1 preamp

Siemems/Amperex 7308-6dj8
Shuguang triple-mica 12ax7 select-Doug's tubes
Mazda/Radio Technique French military triple-mica 12au7
The first pair are expensive.The "select" are about $13/ea and the Mazda tube will run about $60/pr.
Of course there are always the Golden Dragon/VAC tubes.These come "gold-pin "and "standard".The GPs are more linear/more detailed and Stds are warmer.Maybe GPs in the phono and Stds else-where.

I use Mazda 12AU7 tubes from the 60's.Great tube,dynamics galore.
Costly though.I paid $180 for a matched pair.

Check :Quest for Sound ads or a google search will also put you on to the Mazdas.

My original reco was just to help you identify the tubes that you have. I see that everyone else is chiming in with their personal favorite NOS tubes. I have heard them all, Siemens, Telefunkens, Amperex, Mullard, etc. To be honest, they all have their strengths and weakness'. All are much better sounding than current stock Russian/Chinese tubes.

I would say that the Mullard's are the warmest, if that's what you seek. The Telefunken's are also warm, but with a tad more air than the Mullard's. The Amperex and Siemen's offer even more extension in the high's, but are not quite as warm (sweet). Currently I am using Telefunken's in my VAC line stage (and phono stage) and I am quite happy with the sound.

Cheers,
John

PS: according to the VAC manual the four tubes are (2) 12AX7's and (2) 12AU7's.