Hi there. Sorry for bringing this up again, but did some interesting research on this.
As in any other tube amp, there is power tube distortion when cranked, BUT it seems that DT 50 (i guess 25 too) works different, as Master and Volume pots seems to attenaute, and not pushing the tubes harder. So, if you max volume and master knobs you wil let tubes work at 100%. Prescence knob seems to add some power amp distortion when maxed too.
Some people say that you can get tube distortion maxing Master Volume and using Channel Volume to control the overall volume. I did some recordings to try that (as i found no difference at first) and the tone was almost the same (very very subtle difference). Master and Volume knobs seems to do the same work: attenuate the signal flow sent to Power Tubes, but with the difference that Master knob does it in both channels, and channel volume (surpire, surprise) only in its channel.
There is one simple test everyone can do without blowing the windows: set the volume knob of your guitar at 20% aprox and then crank Volume and Master knobs in the amp: yes, its alive!. Tone will be poor, but is the easier way to try it.
I can use my Helix Stomp in 4cable method to lower the overall volume without shaping the tone, and then crank the amp to get that wonderful tube distortion. Guess you can do the same with a PodHD and its Master Knob.
My current setup is a Plexi Bright pre in Voice II without cab simulation and boost on. Speaker set-up is a "3x12" (Celeston G12H90 in the 112 Dt50, and a cabinet loaded with two Greenbacks just below) Tone is just awesome!!! Well, I can't get the same amount of power amp distortion that you will be getting in a real Plexi, but hey, they can't have a fender clean, a vox, and a Dual Rectifier in the other channel ;)