If you want more distortion, you have to increase something. A higher input voltage will distort the first stage. Turning up the channel gain will cause distortion somewhere along the chain, depending on how large the input is. Keeping the master volume down will keep the power stage from distorting, so, if you want that to happen, you'll need to crank the master.
This will, of course, result in a very loud sound coming out of your speakers. There are devices available to reduce the power to the speaker so that you can crank the amp and not be too loud. Doing this will reduce the life of your tubes.
If you're already diming your volumes, and you still don't have enough distortion for your taste, then increasing the input level is the only answer short of modifying the amp. You can do that with a different instrument, higher output pickups, or a clean boost pedal. Clean, so that the distortion is still produced by the amp itself.
