Keep in mind that it all depends on the vintage of the Amperexes or Siemens involved. I tried both white label USA Amperex 7308, and West German Siemens 6922. I would say that there is a significant difference in sound quality, but not one that I would call better. I personally have never been as much of a fan of the Amperexes as some are. I find the Siemens to have a similar overall tonal color (that's not a negative), but a little leaner, with a little finer delineation of high frequencies, and maybe a little faster sounding. The Amperex gave me a more robust sound. Images a little more fleshed out with very stable imaging. But overall, they made the Tjoeb sound a little less modern and a little reminescent of vintage tube equipment; not necessarily a bad thing. Both are very good tubes, and much better than the stock Phillips. My very favorite by a long shot has been the German Telefunken 6dj8. Beautiful, wide open sound, with great tonality, and soundstaging. If you ever look to buy a pair make sure that they are real Tele's with the <> on the bottom. Sometimes they are rebranded Siemens.
Keep us posted.
Keep us posted.