I recently purchased a Firs Sound Deluxe Presence Mk III used, and called Emanuel Go (the designer) to ask some questions as well as tube recommendations since it didn't come with a manual. I'll note here that he is a very soft spoken, but kind and helpful man.
Tis pre uses one 6922 for each channel, and as most of you probably know, good ones can be very costly, and those are beyond my budget. Anyway, when I asked Emanuel for his recommendation, I was quite surprised at his answer. He explained that he had decided to experiment with using 5670 tubes in the place of the 6922 because he had heard the tube used in a DAC designed by a small company that he respects and was impressed. Of course, it needs a tube adaptor so that it can fit. An audio friend of his tried the 5670 in his pre in place of his very expensive (Siemens?) tubes and was impressed.
I found the tubes and the adaptors on ebay for a total of $34.00, and after living with them and comparing them to my mid-grade Dutch tubes, I can confirm that in my system, they are superior to the 6922. There isn't much difference in the level of transparency, as both are very good, but the body of the sound and dynamic impact are markedly improved, as well as a more natural and lifelike presentation.
it's so affordable that no matter how doubtful you might be, there isn't much to lose by trying.
