Best mellow sounding 12ax7 for Thor TA-1000 preamp

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!

Jbrrp1, wish I lived in Minneapolis. It feels like Minneapolis here in St. Louis now. I may try some used mullard 12ax7 as an experiment because right now I need "too warm" to balance the edginess.

St. Paul is nicer .

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!