Sonic Difference between 6550 and KT88 tubes

I'm using 6550s right now in a Cary V12R. I like them, but I' curious to know if there's any sonic difference between the 6550s and the KT88s. They seem to be essentially interchangeable.
I've found that in general (and there many variables) GE 6550 are better in the bass, a little drier with a flatter sounding frequency response, the KT88 being better in the midband with more air.
My experience is similar to Noromance.