Ceriatone OTS. sudden change in tone

[Steven_nl]

Hi Guys

My Ceriatone seems to change tone after it has been switched on a few minutes. There seems to be a (quite sudden) drop in highs.

What would you suggest?
I don't have to much experience with all tube amps. I realise an amp will change once the tubes heat up, but this doesn't seem quite right.

thanks!