Cranking the DT master volume allows for more headroom (only really required on clean tones) but if your preamp (POD) input volume is low then you will not see any real benefit from this but may experience increased noise and hum (typical wen you turn the master volume on any amp to 11!).
I do not think that this is the case, and would appreciate it if you can provide some concrete observations as to why you think it so.
Consider the following: the FX return is always amplified to the amps max volume, irregardless of the position of the dt master (which can be fully down!!), so the signal chain is (at least):
preamp (pod or dt)->dt master (attenuating a ~line level signal)->fx send->fx return (still line level)->power amplification->speaker
You can check this if you like by monitoring the level on the fx send.. it will go up and down as you turn the dt master volume up and down.
So, what I think is happening here is that the amount of power amplification is never changed by any of the controls. The power amp just takes whatever signal it receives, and amplifies it to the max. If the signal the power amp receives is low (master volume is down) then it amplifies cleanly. If the signal it receives is hot (master volume is up a ways) then it will start to distort and distort more the hotter it gets. So I guess the headroom is fixed at a particular decibel level, and has nothing to do with the position of the master.
Of course, there may be class A and A/B differences, and there can easily be a degree of digital tube distortion being applied at any point, so I don't know the above for sure. But there are some easily observable facts mentioned which I encourage you to consider. Also, if I have made any errors or my ideas can be refuted, please feel free to do so in the spirit of understanding this thing.
I am _so_ going to hook up a sine wave generator soon and measure the total harmonic distortion levels at different positions of the master(s). I'm moving out of this apartment in June and will relocate my studio from living room to garage and can then conduct tests with the amp cranked. Can't wait