Best mellow sounding 12ax7 for Thor TA-1000 preamp

I just bought  Thor TA-1000 and am very impressed with the resolution but want it to be more mellow sounding... Darker and less treble emphasis. Can anyone recommend the most musical but mellow/organic 12ax7 for the Thor? When I bought it the unit has telefunkens. The outputs are already mullards so I am focused on the inputs.

It is feeding vtl mb-185 signature amps and quad esl63 with vandersteen 2wq subs.

Stick w/Mullard.Anything NOS,ESPECIALLY 1950's Long Rib Plate..Holy Grail would be to find the pretty much non existent Western Electric,also from the 50's...
May I suggest that you look at 5751 tubes. Slightly lower gain but much cleaner.
They are a direct replacement.
Google "Joe's Tube Lore" for a nice review of many different brands.
While your and my results will vary from those of Joe's, it is a good general starting point.
You could buy several pairs and try them to find what best fits your system and then sell those that don't float your boat.
A very good one with a bold presentation and rather sweet treble is a Raytheon 5751 with the windmill getter.
Good luck and Have Fun with it.
New Production Mullards can do what you want quite nicely. On a budget that is what I would (and do) do.
NOS RCA 12ax7 cleartops would fill the bill. As would the Raytheon 5751's mentioned .
I have to agree with vintage Mullards (haven't heard the new Russian ones) and 5751 especially the RCA black plate.
RCA 12AX7 or 5751 BlackPlates have smooth, open sonics. A classic tube sound, less expensive than Mullards.
For modern production jj ecc83s have a darker tone.  I have a pair of vintage RCA 12ax7a that are very smooth, they have good overall balance, not bright but still have good high frequency extention. ...    
I have used Full Music 12ax7 in my TA-1000 for last three years and there is not a hint of brightness. They seem to pair well with a pair of cryoed Mullard 6201 that I also use in the unit.
(RCA "clear tops" generally refer to their 12AU7's, 6201's are a substitute for 12AT7's) ...since the Thor TA-1000 has plenty of gain, I use black plate 5751's with their lower gain in mine. Telefunkens are generally thought to be "brighter" and better for older vintage equipment.

-  As an aside, a knowledgeable friend compared black plate vs. grey plated tubes and could not hear much of a difference. 

Ha. Knowledgeable friend he have cloth ears! All tubes have their own unique sound. E.g. Sylvania 5751 Gold with grey plates sound a world away from GE black plates. I second JJS for a warmer darker sound. You will lose detail over the Teles  though.
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 get to the truth of the matter.
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!
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!
NOS Telefunken ECC 83s
and if you really want to break the bank ECC 803 s

Andy at Vintage Tube Services is without a doubt an incredible source, impecebly knowledgeable, the most refined  testing, my highest recommendation.

Thanks for all the great advice guys! I tried two NOS Mullards and it helped quite a bit. Then I changed by DAC caps from Dueland back to Audio Note coppers and that helped even more. A cable change or two and I should be good.

Hi, i just happened to stumble across this thread. I recently aquired a Thor TA-1000 mkii preamp. (at least i think it's a Mkii) The serial number is greater than 1000, and I read that if that's the case then it's a mkii?? Is there anywhere i can get an owners manual for it, and  If not does anyone here have one that they could make a copy for me? I would be willing to pay for your time and of course the mailing cost. One of the questions I have is what is the toggle switch on the back for? it isn't labeled. thanks for any help! 
Probably not very useful but my mark I serial number is in the 200's. I dont know when the mkII started.

The only switches I could imagine would be either a USB/rca input switch (does it have a USB input?) or a phone mm/mc switch, if the preamp is a ta-2000 with phono.