Good 6550/KT88 tubes

Hey folks,
Can someone tell me what the sonic differences are between the Svetlana KT88 and 6550c tubes are? Also I heard that the Tesla KT88 tube is one of the best production KT88's out there? Any comment? I am looking to tube my Rogue Magnum 120's and could use the help. Any tube suggestions would be appreciated.
Thanks guys
dudeaudio