There were some Harman Kardon amps that went into protection mode if the preamp was giving some DC to its inputs. I can't remember model, or how many years back I heard about this. Maybe trying another preamp (no preamp if you feel safe) to see if this can be a problem. I can't remember if this happened right at start-up, or after the delay (few seconds?) turned the amp on, then went into protection.
