Sorry to be a pain in the ASS about this subject, BUT...
I just got back from band practice. the 2 tube approach is not gonna work. For reasons I don't wish to get into.
If I get my amp modified and add a master volume, will I be able to crank both volumes to the max and lower things with the master volume? I know this will lower the value of the amp.
crank both volumes and lower with the master? I may be misunderstanding but to me that sounds like you already have a master volume control.
I agree with Tim. Buy an attenuator. That way you can crank all you want without a mod to your amp and if you ever decide you want to sell the amp, you wont have lowered it's value.
There are a lot of different manufacturers making good attenuators. The one I own is the weber mass lite. it works well, sounds good, and is decently priced.
sure you can add a master vol... but are you sure that it's the pre-amp overdrive you want and not the output tube distortion.... way different sounds...