Audio Research SP-10 6922 or 6DJ8

The electrical characteristics are close enough that the 6DJ8, 6922 and 7308 will all three work in the ARC circuit. There is one other possibility, the 7316, which is very low noise military version, also electrically similar to these other three. If you want an ultra clean and linear sound, try the USA Amperex 7308, especially the CEP version. Somewhat less linear and more warm would be the Dutch Bugle Boy 6DJ8, or the Mullard 6922. The Siemens 6922 CCA is a very detailed and low distortion tube, but is less friendly to bright recordings with the ARC. You did not say what you were listening to right now, and if you were happy with the tonal balance and dynamic range provided. I and others here can probably advise you, given that information, and what you like and dislike about the sound right now.

Frap, the Tele ECC88 is a warm tube, I think you may have suffered a slow loss of resolution and bandwidth over the years, and are perhaps forgetting the sound of those original (supplied) tubes in your ARC. I agree that just before they departed this earth, they probably were very dark and in their own way, beautiful. An easy and inexpensive way to find if this sound is really what you want, is to buy some tubes at or near the lowest usable transconductance. A tube dealer would be selling you VERY used tubes and the price should be remarkably low. I suggest Telefunken, as the midrange is still beautiful, until it dies. I do agree with your comments about the Sovtek 6922, it sounds remarkably bad when compared to at least a dozen other possible candidates. You mention that the line section tubes "were" also bigger. Does that mean they were replaced at the same time? If so, you have two things going on at the same time that effect the SP10's tonal balance. Have you checked all the other tubes? If you are having to replace this many tubes in the output section, how about those in the power supply? If power supply tubes are bad, and you put in new 6922's, that would really provide a situation for poor sound.

The topic of tubes has been covered here many times, but to answer your question, the Russians have done a great job on some tube types and not well at all on others. Unfortunately, the tube you and I are discussing ( 6922 / 6DJ8 / 7308 ), has not seen quality manufacturing by anyone that can equal the best of the new old stock. There is a huge demand for this tube type, and the need grows every year. Perhaps a US importer or distributor will invest the money needed to get a break through in quality on this particular type. I suspect the Russians are currently building the best tube they can, given the budget, tools and materials they have to work with.