I am not sure how the EH6550 qualify as more reliable, Svetlana tubes are pretty bulletproof.
EH are made by Sovtek, I believe, and I have always found Svetlana tubes to be of a higher quality, both in build and sound. EH is Sovtek's go at building tubes which will compete with Svetlana and JJ, as most find Sovtek to be a step down.
I apologize for not knowing the whole Electro Harmonix history...
However, I do know the sound of both tubes. If you are looking for a more drier, more neutral sound from your Sonic Frontiers amp, go for the EH6550. That would mean if you find the SF amp overly warm or lush, which many people do not.
You state that you are happy with the sound of your amp, if that is true to the point where you do not want to push the sound in cooler direction, definitely pick up another set of Svetlanas.
And, if you want a bit more sweetness and bass impact, try JJ(Tesla/Teslovak) KT88. But, I do hesitate to give you a recommendation of this tube as there was a production run in 1998 which was awful. Apart from that group, the tube is reliable, but you never know when you are buying tubes, and I doubt that the JJ KT88 in general is as reliable as the Svetlana.
GOOD LUCK in your search
EH are made by Sovtek, I believe, and I have always found Svetlana tubes to be of a higher quality, both in build and sound. EH is Sovtek's go at building tubes which will compete with Svetlana and JJ, as most find Sovtek to be a step down.
I apologize for not knowing the whole Electro Harmonix history...
However, I do know the sound of both tubes. If you are looking for a more drier, more neutral sound from your Sonic Frontiers amp, go for the EH6550. That would mean if you find the SF amp overly warm or lush, which many people do not.
You state that you are happy with the sound of your amp, if that is true to the point where you do not want to push the sound in cooler direction, definitely pick up another set of Svetlanas.
And, if you want a bit more sweetness and bass impact, try JJ(Tesla/Teslovak) KT88. But, I do hesitate to give you a recommendation of this tube as there was a production run in 1998 which was awful. Apart from that group, the tube is reliable, but you never know when you are buying tubes, and I doubt that the JJ KT88 in general is as reliable as the Svetlana.
GOOD LUCK in your search