Subway Blues hissy

[ficelles]

I have a hissy Subway Blues, single input version but no half power socket. FX loop has been bypassed (is that mod documented anywhere?), hum isn't bad, it's just hissy. Have tried different tubes throughout and nothing changes it. Could it be caps need replacing? This amp is relatively new to me but I had one years ago that I don't recall sounding like an angry snake...