Well, more fiddling, without success. I'd swapped back in my PM6002, which is quiet as a mouse.
Back on the K2, a strange thing - as I said, the hum is audible through the right hand channel, including headphones. But when I put the amp on standby, the hum through the headphones gets much louder and deeper, and comes through both channels. Then after a few seconds there is a sound like an air raid siren starting up, back in the right channel, then a few seconds later it sputters out, although the louder hum is still present.
All quite odd.
"His skills are getting faster with Grandpa the master." - Jonas Brothers