Walter, you are dead on. The stock tubes are matched and balanced to each other very nice, but its hard to look back once you put some KenRad black 6SN7's in there. I've got 4 matched pairs, all primo NOS's in original boxes and they vary on noise. With that said, all spit in my face for the first 100 hrs and still can get cranky now and then. Basically the Supra is so discerning that you are hearing the tube's operation and with such high caliber stuff verything must be dead on. Its not that the pre is unstable, its just very very good and will tell you about a tube's given quality. Its a pain if you want to mucho magic, but as walter says, i think its worth it because once you get them right, they are stable. I've tried the Brimars (no noise, but too precise for me), Sylvania VT231 (alot like the black Kens, not as enveloping or expansive, but relatively quiet) and the ones I'm using now, Tungsol round plate black glass, which on this smaller system I prefer (sweeter, better micro-details on chestiness, guitar body, etc., not as expensive in depth field as Kens) and with very low noise.
I bought some tubes from a good dealer (Sylvania green letters) and they were so noisy I traded them back in for some RCA 12BH7 inputs on my amps. It was in the beginning and I thought, oh my God, I'll never get the noise down in this pre - but it wasn't the pre, it was the tubes.
You are getting a custom handmade $10-15K pre for $2500-5K and at that level things become exacting - its part of owning a Lamhorghini. Be patient, but know that it will never be a Spectral. Its not perfect, and should be kept standard, but if you get it right its very stable and will last you twenty years or more.