Depends on the circuit they're in, of course, but in my JA80s, the 6550s are much leaner and harsh, although they may have slightly better low bass extension. All in all, though, the KT88s' midrange was much fuller, realistic and harmonically correct; a much more refined presentation compared to a coarser, more rough around the edges sound from the 6550s. I'm comparing the best 6550s I've had (ARC Philips tubes from the 70s and 80s, not Tung Sols, which may be the best 6550s out there) vs. GEC Gold Lion KT88s; can't tell you how Svetlana 6550s (which I have heard and would rate close to the Philips) would stack up against that company's KT88s.
