I'm new to tube amps, and I've recently been trying to take people's advice and crank mine up to get some saturation going, but I've noticed that when I crank either of my tube amps up (Marshall Origin 20C, which I love otherwise, and a Blackstar HT40 Mk II Black & Blue, which I'm lukewarm about), I get a lot of hissing and buzzing coming through. Different guitars don't change it. Even when I use my strat with noiseless pickups, running into a noise gate on full, I can still hear that dang hissing and buzzing, and it's basically equal in both amps. The Marshall does it regardless of if anything's plugged in, the Blackstar starts up as soon as I plug a guitar in.