Musky1963:
As you look at the front of the 99Magnum preamp, the two tube sockets on the right are the amplification tubes. They are the most important for having low microphonic tubes. They are also the two that will have the most effect on the sound. The two tubes on the left are the mu-follower, or driver, tubes.
Steidlguitars was correct when he said that the choice of tubes will be system dependent.
I tried RCA in the amp side, Sylvania in the driver side: a little too much sibilance in the system for me.
I tried Sylvania in the amp side, RCA in the driver side: way, way, way too much sibilance in the system for me.
I ended up with all RCA tubes.
Sold the Sylvania tubes to a guy who comes into our local shop and who has a darker sounding system - he loved them... and subsequently bought two more for the driver side... still loves them in his system.