Don't know the answer to your question on comparisons, but I run Svetlana 6550's in my Sonic Frontiers amp and love them. They are detailed, and smooth. In the SF, they don't have a strongly "tubey" sound because of SF's design. They produce a lot of bass, as well. I'm very pleased the the 6550 tube. The KT90 can be used in SF equipment, but require careful biasing, according to the manufacturer's tech, so you might be cautious in setting them up if you try them. I'd be curious to know how much you like them, if tried. Sovtek has had the KT88's for quite a while, but I've been so pleased with the 6550's that I haven't "rolled" them. Good Luck in your quest for info. I'll be watching. Stu
