Ilbiffo,I use the Svetlana KT88's in my power 3 se's and they sound very good. It is my understanding that the 6550 tube is in general a more reliable tube but the KT88 produces more power and has a somewhat warmer sound. I agree with Kevziek in that all tube amps will require periodic attention due to tube failures etc. The tube that sounds best will depend on the other parts of your system and listener preference.
