Best tubes for Sonic Frontiers 80

Tube types will depend on what tonal qualities you're are looking for and will depend on your ancillary equipment as well as the speakers you are driving. I've used 99A's, KT88's, 6550c's and EL34's. They all sound different. I ultimately chose SED 6550's and EI 6dj8 Elites which seemed more balanced and neutral in tone. YMMV.

Jefferybehr, The KT99a is a KT90 in disguise. This is the original tube Sonic Frontiers used in the SFS80 and the SFM 160's. It was made by Yugo and relabled by Gold Aero. I found its tone powerful but a bit on the cool side of neutral, even after changing out the small tubes which I believe were relabled Siemans. FWIW I agree with your comments re EL34's and that is about how they sounded in the SFS80 (except for Tesla E34L's which were also relabled and sold by Gold Aero. These were, for lack of a better discription, sort of 'technicolor' and did not lack high frequency energy. Since I'm talking about my observations of 'tone' with the SFS80 (and a couple of other amps) I've just never been a fan of the KT88. The upper mids and highs are excellent but I have not experienced a similar smoothness and balance in the lower mids and bass. But then I've only used a couple of current production brands including SED's.