Best mellow sounding 12ax7 for Thor TA-1000 preamp

Yeah, I was a bit surprised by "no difference between black plate vs. grey plate", too.  I hear very clear differences between Sylvania black plates and grey plates; same thing for GE black plates and grey plates.

For the preamp adjustment the OP is addressing, I would think 5751 RCA Black Plate triple micas would be a nice fit.  I find them to be richer, but still have good enough detail and extension.

Ok. I stand corrected. That's what these forum are for....to get to the truth of the matter.
Cheers,
Truman

I own a Thor TA1000 Mk II and its one of the best preamps I ever owned. I did have to do some tweaking to add some warmth but at no time was it ever bright or harsh. It's very detailed and with excellent separation and air but to add some warmth I am using Brimar 12AT7s and RCA 5751 Triple micra Black Plates. also add a good power cord, I am using Cardas Golden Reference. Also Amperex 12AX7 Bugle Boys are a good choice. Also a good choice for the 12AT7s.
Remove the Telefunkens.

Thanks for all the great responses. Gjrad mentioned the 5751 as others have. Would a 5751 be warmer on top than perhaps a mullard 12ax7? If so, I would actually welcome less gain/volume

You sure cannot lump all 5751's together.  Try some GE grey plates and you will get a leaner presentation.  Try some RCA black plates and you will get warmer.  And there are a whole lot of variations in between.  I do think "Joe's Tube Lore" does a pretty fair assessment of the options available, and is worth your reading (it's in the FAQ for Tubes in audio asylum).

I compared RCA 5751 triple mica black plates to Mullard CV4004 tubes (12AX7 equivalent) from Upscale Audio in the input section of an EAR 890 and I found the Mullards to be too closed in on top, too warm, and a bit "fuzzy" in focus for my tastes (the EAR 890 is a warmer sounding amp to begin with IME).  If you live in Minneapolis area I could loan you a bunch of different 5751's to roll - - it's a lot of fun!