These are the standard Svetlana EL34's. I've always liked the EL34 sound in vintage amps (liked it better than 6L6 types.) I haven't had any other brands to compare in this amp. I did have some JJ's in a vintage amp a year or so back and thought they were nice tubes. Haven't heard any others. This new amp is in a completely different league from my previous vintage amps, so can't compare.As well as EL34, KT88 and 6550, there's also 6CA7 and KT90 to consider. If no clear winner comes from these discussions, I'll probably stick with EL34 as a known quality. Leaning toward JJ's. The JJ EL84's I had running in a Scott integrated were substituting for 7981's and held up great. That's quite a feat.
I've never heard a KT88, and only heard 6550 in guitar amps, so don't know anything about them. Heard a pair of 6CA7's several years back, but was to long ago and in old vintage amp, so can't make any meaningful conclusions there, either.
Dave