6550 vs. KT88: What's different about the sound?

I have a Music Reference RM9...great amp. I used EL34's for several years...nice match for the slightly agressive ribbons in the Newform (best imaging I've ever heard) Now that I have the Maggie 1.6 QR, the 6550 (Svetlana) sounds better... but will the KT88 better define the high end without being aggressive and better control the bass without being anemic? KT88's are much more expensive than 6550's so before I lay out an extea 300 $, or so, I'd like to know what to expect. Any help out there?? Better to get me at [email protected]. ( use "Tube question" as the subject so I know it's not spam )