I own a Thor 1000 with a Khozmo attenuator and V-cap upgrades. The unit came to me with Russian 12AX7 and 12AT7s. These sounded terrible. So I changed to a pair of Telefunken 12AT7s and Raytheon Windmill 5751s. The old stock tubes were much better however I really think that an old stock 12AX7 like an Amperex would be better. I think that the lower gain of the 5751 robs the Thor of a bit dynamics making the Thor less concise. If you want to tone the Thor down a bit I wouldnt suggest Telefunken AT or AXs. My favorite 12AT7 are Amperex.