I've got the Chinese Genelex copies (KT88's), early 50's RCA JAN 6SN7GT's --- and the Siemens 7308's just brought the whole package together. Specifically the results were:
KT88's: Gave me more sustained power during heavy transients on both ends of the spectrum, but most notably on the bottom end, where it was tightened up dramatically.
RCA 6SN7GT's: Widened and deepened the soundstage, while at the same time bringing the musicians into a GREAT pinpoint focus. The stock Sovteks were all over the place.
Siemens 7308's: Took away the glare/smoothed off the top, which is what I wanted, and at the same time even further tightening of the bottom end resulted.
I used the same two CD's every time I made changes. All tube changes were made after I had already gotten the CD-303. The difference between the CD-303 and Ah! 99' was the most dramatic thing I did to this system in the last two months. Second would have to be the 1951 RCA 6SN7GT's.
My speakers in this system are Silverline SR15's on shot loaded stands and I'm using DH Labs (8') Q-10 biwired.
I'm still waiting on my Soliloquy S-10 to be delivered, which will finish this office system. Seems they made some changes in the amp for the sub at the German factory and are having to retool all the boxes. It's been a two month anguishing wait.