I was a Line 6 tech for years. I don't know what model you have, but I would guess the power amp chip blew. Why, I have no idea without looking at it.
Repair of most L6 equipment consists of board swaps. If I had it on my bench, and I determined it was the power amp chip, I would remove the chip or the board the chip is on, then power up and check all voltages. If OK, I'd replace that chip or board. Should be good to go.
As new models come out, Line 6 uses up the parts they have in stock on older models. When the boards run out for discontinued models, repairs have to be made on a component level. Depending on the amp or the breakdown, this can get expensive when the amp is out of warranty. I had a few Flextone II heads in the shop that were devils to fix and took a lot of time. Had boards still been available the repairs would have been 30 minutes.
I've been repairing amps for many decades. I have no idea what "kettle leads" are.
If your amp is out of warranty, and you have a local tech who would be willing to look at it, e-mail me and I'll get you the service info the tech might need. I'll need to know the model number. firstname.lastname@example.org