I use the KT-66 in my Two Strokes. I just love the”beef” I get from them. And I use JJ’s for those. I’ve used the 6550 on some other amps that I’ve built and love that tube. JJ’s there as well.
On our Renovo Tejas amp, I tested around 15 different 6L6 tubes. Everything from GT to JJ. And I landed on Tube Amp Doctor for our 6L6’s there. They just fit the bill best. There can be a huge swing of difference between tube brands. What I have found, is that JJ seems to be the most consistent and well priced across all their tube offerings.