I am sure it is the stepped attenuator. I have switched tons of amps into the system just to have fun
and the preamp, right channel, is consistently the culprit. It is a very loud and destructive sounding pop! The cleaning worked temporarily as mentioned by others.
I do love this preamp and after five years I retain the
original tubes. I've bought others brands to play with but the microphonics were horrible and am going to have to bite
the bullet and spend $100+ at AI or Kevin Deals to retube this.
I am hesitant to do all of this because I pre-paid for the preamp factory direct via my employeement and it took AI a year to deliver and at times I wasn't sure if it would ever show up.