I have owned a pair for a year now. Excellent amp for the money IMO. From my trials:
For the best sound, Classical or Jazz, a pair of RCA Cunningham single plate 2A3's with an RCA VT-231 (gray glass).
For general listening, the RCA double black plates (the ones with the "R" and transitor symbol - 1940 to 1944 manufacture) with the Sylvania VT-231.
For rock, the stock Sovteks with the Hytron (pre CBS) 6SN7GTA.
For progressive jazz, female vocals, any 2A3 with the RCA 5692 red base.
Didn't like the Ken-Rad 2A3 or 6SN7, though. And, it really did not make a difference to me which 5Y3 was used. The stock Phillips works very well.
Of course, all of the above is just the way I hear 'em.
For the best sound, Classical or Jazz, a pair of RCA Cunningham single plate 2A3's with an RCA VT-231 (gray glass).
For general listening, the RCA double black plates (the ones with the "R" and transitor symbol - 1940 to 1944 manufacture) with the Sylvania VT-231.
For rock, the stock Sovteks with the Hytron (pre CBS) 6SN7GTA.
For progressive jazz, female vocals, any 2A3 with the RCA 5692 red base.
Didn't like the Ken-Rad 2A3 or 6SN7, though. And, it really did not make a difference to me which 5Y3 was used. The stock Phillips works very well.
Of course, all of the above is just the way I hear 'em.