Yup, Spaceati is right (as usual).

What I do for my DT25 Channel A (using a Clean Fender amp NFL 1) is dime BOTH Master volume and Channel volume then add "Drive" to get the necessary level I need. In essence then, the Drive setting on Channel A is providing the master volume level. Works for me. I don't find with the Fender Amp (NFL1) that the Drive adds noticeable crunch. I am usually at ~1 o'clock'ish + for the Drive.

On channel B, I set it for the desired "crunch factor" .... For my needs, I normally use the AC30 setting (NFL 3) for grunginess. The Drive level determines the amount of "grungineess" and then the Channel Volume is used to match the volume I have on Channel A. I think it is normally somewhere in the vicinity of 11 o'clock'ish with a fair amount of Drive level (sometimes max). Obviously for the other NFL's (2-3-4) you will need a much more reduced Channel volume to equate to the Fender's level on Channel A.

A footswitch lets me easily go back and forth between the two channels and I adjust the B Channel Volume to make the transition between them work appropriately. I always dime my Master Volume on the DT25. A foothswitch changed everything for me and I can't believe I waited so long to get one. You'd think L6 would include a footswitch with the DT amps??? ... old Leo Fender got it right by doing that from the beginning.

I really like the "Classic American Clean" so that is my stock reference. At first, I had a hard time getting the level I needed until I realized that the Drive doesn't crunch it much, and can be used for extra gain with both Channel and Master volumes dimed.

I also have a HD500 but currently I use it strictly as a Multi Efx pedal and use the DT amp's internal preamps. With an Amp footswitch you can use both A & B channel's preamps even if the 500 is set for "no amp model". A little trick I learned after the fact.

My 2c for what it's worth.