Help diagnose amp noise

If, at the same amp settings, there's more noise with a guitar plugged in than without, the noise is coming from outside the amp (guitar, pickups, effects, etc).

If you have flourescent lighting, single coil pickups or are sitting too close to the amp (or other electrical noise source) you'll get more noise than if you don't.

 

Try different guitars for certain, and also try the amp in a different location to see if it changes.
Another trick is to turn the treble up pretty good, and change the bass knob to see if it increases hiss. If it is high frequency hiss but the bass knob increases it, this gives a bit of a clue that it could be for example a bad cap in the eq section.
Also check your verb. Turn the verb down and see if it goes away. A weak verb tube will make a lot of noise and it is easy to forget to change the verb while testing, many guys set their verb once and leave it be.

You are always likely to have some hiss at high volumes. A bad tube will sometimes hiss.
If it has more than one channel, set the gain on the clean channel high and the gain on the dirt channel low, so you are getting closer to the same level of overdrive from them, and compare the channels to see if it is one channel that is more.
That again doesn't tell you a lot but it does give you an idea.