There are a number of ways to attack the issue.
First off, you want to make sure your guitar is properly grounded and shielded. You can google "quieting the beast" for a very good website on the topic.
Make sure all of your cables are good - sometimes a bad connection can cause grounding issues.
Only use as much gain as you absolutely need.
Don't play close to your amp, a computer, or even many lights and appliances. They all throw off a lot of electromagnetic noise, which will get "picked up" by your pickups.
If you haven't changed your tubes out in a few years, it might be time to do that, as well.
Sometimes dirty power can be a problem, or a bad power strip. Try plugging it into a surge protector, or if it is now, straight into the wall. Tube amps have a lot of power filtering circuitry (usually) due to the nature of what they are, but dirty power is still dirty power, and that can cause a significant amount of noise and hum.
Beyond that, assuming the issue is indeed your amp, it might be time to take it to an amp tech for a proper diagnosis and repair. Sometimes the issue can be as simple as needing a cap job - the filter caps are sometimes not beefy enough, or are going bad. Could be a short or ground somewhere.... hard to say.
Good luck!
Saul