I don't have a sonic frontiers amp, but a Copland and the Svetlana's have worked better than any other power tube I have tried including Sovtek KT88, Svetlana 6550C, JJ Tesla KT88, Tung-sol 6550, and a GE6550A. The mids were more pronounced and very silky smooth. The bottom end was the tightest of the bunch.
I would only think SF was right and it was a bad batch. The KT88 was their newest tube and there might have been some growing pains. They work well now. If you go to a few of the tube dealers web-sites, that is the tube they are using personally. I have had SF amps and they should work fine.
I would only think SF was right and it was a bad batch. The KT88 was their newest tube and there might have been some growing pains. They work well now. If you go to a few of the tube dealers web-sites, that is the tube they are using personally. I have had SF amps and they should work fine.