Loud crackling in RP65 112

Aaaagh! Please help... my normally stunningly reliable amp has become unusable. It coincides with me accidentally leaving the speaker switch on 4 ohm :-(
Is is possible I've cooked the transformer? It takes a while for the crackle to develop, and it really kicks in when the amp is cranked up. The tubes look like they're ok, both glowing the same brightness.
Any suggestions?
Best wishes,
Tim

Try a different speaker.

Thanks, but tried it with an extension cab. Same thing.
Got some new valves, gonna give them a go and see if they help.

Try cleaning and re-tensioning the tube sockets when you put them in. There's a lot of miles on these amps and a little corrosion goes a long way. Mike.