Do some research on this in the Audio Asylum tube forum, and/or post this request there. You have a better chance of getting some good responses. IMHO the design of the amp is more important to the sound than the tube's designation. However KT88's, 6550's, and KT 90's, for example are large power tubes and unually produce up to twice as much power, ergo when a manufacturer wants to build a high power amp they choose these tubes. When high power is not an issue they use EL34's etc. The Primaluna is an exception and I think that the use of the KT88 may produce a tighter and more powerful sound even though the power delivery isn't that much different on paper. Why not call Kevin Deal and ask.