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.

Surf, the Ei KT90 type 3 is a VERY-good-sounding 6550/88-type tube and was voted the best-sounding new-production 6550/88-type tube in the most-recent 'Vacuum Tube Valley' test. I've used them in c-j Premier Elevens, NewGen 94s (2 pairs each amp), and Quicksilver V4s. Mike Sanders of QS recommends them but recommends against the EH KT90, as does Jim McShane, http://pages.prodigy.net/jimmcshane/tubes.htm .

In spite of the several recent (say, this year) comments I've read about plate-structure misalignment and subsequent glowing-red plates, NONE of my 18 glow anywhere but the filaments. I've recently bought 4 new matched quads from an eBay seller whose e-mail address is JSPHT8338@aol.com. His eBay BIN prices were $104, and his non-eBay price is $100 per matched quad. You might try him--great service, too.

EL34s are known for their harmonic richness and sweetness in the midrange, but usually they're soft in the trebles and less controlled in the bass--at least in an amp that can use 6550/88s.

Newbee, what is a '99A'?
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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.