Hi Jon, im guessing you are my mysterious benefactor on my website who's helping me troubleshoot the amp? Scott - thanks for the tip! They've been recommended to me as well before - I want to use it as a bit more of a learning experience though, hence all the messing about Up to now with jon's help, I've ruled out the preamp stage as a hum source and checked the sockets. The replacement at7i got is also also of the wrong type so there's another on its way. If anyone is interested I can update this as I go along.