Yeah, the best source of getting a good answer would be to ask ARC but they don't take phone calls any more and don't respond to email as fast as the good old days. Why would I want to move away from KT150's? Well, they are somewhat sterile sounding compared to the Tung-Sol or Svetlana 6550's for starters. So my question is whether there is anything about the filament current or such that would prohibit using these lower output tubes or KT88's. Anyone know?