Of the 6550s I used in my old JA80s (from around 1993, had them over 10 years), the best of current production tubes were true Svetlanas (not Sovteks). They had some of the midrange of the better KT88s, though could not match them in that area, and had the best bass control of all the tube types I used in the 80s. They will give you most of what these amps can offer. The best 6550s ever in my amps were the Tung Sols, but you have to make sure you get the real ones, as there were a lot of phony ones out there in the market. And the best tubes I ever had in my JA 80s were NOS MO Valve KT88s (GEC Gold Lions, etc.). They are pricy, but lasted four times longer than any 6550 type tubes in those amps, had the midrange of EL34s (which sounded great in that area while they lasted) but much better bass, and seemed to be just the perfect tube for those amps. I did not like the KT90s in my amps--felt they were cold and lifeless, became stale-sounding after a while. Anyway, I see you got some Svetlanas, so you've gone the right direction, in my view.
