I have tried both KT-88 (from Kevin Deal through the grapevine of Audiophile friends) and 6550C -- I think KT-88s are nice, but I prefer the 6550Cs. I have went with warmer / lusher tubes in my pre-amp, and I think that the combo was too much. It all depends on your system & ears. The KT-88s will work, I have heard that they may be less stable than 6550C, also be sure to turn the bias WAY down before inserting them -- have heard of an occasional screen grid resistor blowing (also true with 6550s). Good luck!!