Marshal vs 8080 hissing

[Tahoeblues]

I have a Marshal Valvestate 8080 that recently developed a loud hissing noise in both channels. It seems to get louder as the gain is increased. It still seems to have plenty of volume but the tone is pretty flat and the hissing noise is very loud. The precipitating event may have been that I accidentally left it on for several hours. I have done some minor repairs to it over the years, re-soldered the input jacks and things like that but I haven't replaced the valve or anything. Does anyone have any ideas on what it could be and how to fix it?

[ChS]

I have a Valvestate 8080 which has had issues with electrolytic caps going south, two of them had bulged and were leaking electrolyte out the top. You might want to pull the chassis and take a look around for similar issues.