Thanks for the suggestions - especially for dealers who can be trusted. Based on the current price for the Sovtek tubes, about $12 each with maybe a $5/ tube charge to build a quad, I would imagine that there are a lot of them. The concern I have to have answered is more more one of not wanting to disturb the immaculate flat frequency response of this unit, which was achieved by shipping with and selling Sovtek 12AX7 tubes if I substitute similar but not quite identical replacements. The problem is that I have not taken the time yet to understand how RIAA decoding is accomplished in the first place, and what role the tubes play. It occurs to me that there are a lot of tube data and measurements when looking at the individual tube descriptions on the sites you all have suggested, and they vary a bit for different tube models. It would be good to know which values were the ones the designer focused on when choosing Sovteks. Then I would be able to look for similarity in the tube catalogs. Unfortunately, Thor seems to have folded their tent at the end of 2014, and their web site has no referral for support.
Sampwalker: My apologies for misusing terms. As I explained in this post, I am totally uneducated about how a phono-stage circuit works; I assume that the Sovteks are used in the decoding of the input signal and the 12AT7 tubes are used in the gain stage to bring the decoded signal up to line-level. I guess I've got some reading to do.
Rzado, I have an early TA-3000.It had been a dealer-owned store demo unit at the legendary Bruce Jacobs' Salon I Audio in 2000, listed at $7500 and I have no clue as to what I paid for it and never have. Bruce had store poilcies that were generous for trading up and I bought it as part of a deal where I traded in a Vendetta research phono stage and a VPI TNT table with a tricked out ET2 arm plus around $5k for the Thor and a gently used Versa Dynamics table. The table died shortly after both Salon I and Versa went out of business and I pouted for ten years and just listened to CDs because I couldn't afford another table that good until a couple of years ago when I bought another record player. So, the Thor was practically NOS in 2012 when I put her back in service. I had a Benz back when I first bought it; can't remember the model, but it was one of their top models. It had only about .25mv output, but the gain on this unit is 68db. I never had a problem with noise in that system (Atmasphere Music Amplifiers)and a DR6 used as a linestage only. I am currently using a Grado Statement 1 (.5 mv output) and the sound is glorious. With 90 db speakers, I reach the threshold of pain at only about 30% of volume control range, but when I run the Thor outputs directly into my headphone amp's pre-ins, I need to go nearly to full gain and it is almost as dead quiet as when throttled back in the main system. I think it's a match made in Heaven for the big Grado.