@agedhorse
1. Is the amp quiet without using any pedals?
Yes, the hiss is loud without any pedals. With the compressor on, its much worse as you mention, but direct into amp, the tube channel has loud annoying hiss.
2. If you are using a lot of treble boost, or if you have the tweeter attenuator all the way up
Both treble knobs are off completely. My PJB cabs dont have tweeter attenuators, although they may have tweeters ? Not sure on that - they're PJB C4 - 4x5" - 2 cabs
3. If you are using an active bass, be sure that your battery is good. Some active electronics packages can get very noise as the battery voltage falls.
Bass is passive
5. Is the amp quieter with the tone shaping switches disengaged?
No
6.With nothing plugged into your amp except the speaker, is the amp quiet with the channel gain, channel volume, master volume and eq all set to the 12:00 position? When switching between the tube channel and the FET channel, it's normal for there to be a just couple dB more noise in the tube channel.
The only way to stop the hissing is to wind the channel volume off or master volume off. Doesnt matter where the other knobs are. But that was yesterday
Tonight, hissing has almost completely gone. Maybe it needs warming up ? How annoying - wonder whats going now now ?
Where do you typically operate the controls?
I'll be back with a photo - much easier