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.

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!

St. Paul is nicer - - I went to high school there.  I live just across the river from St. Paul, so I really get to enjoy the best of both now!