Try taking a 1/4 out from the pod and setting it to output line level and feed this into the effect return of your dt25 (the master volume on your POD should now control the overall volume (effectively bypassing the pre-amp section of the DT25). If the volume seems ok then its sounds like the pre-amp part of the DT25 is faulty and the power amp and the valves are ok. Try plugging the guitar directly into the effect return of the DT25, turning the volume knob on the guitar down first, and slowly turning it up just to verify that the power amp is working ok.
I had this problem and despite being returned to Line 6 service twice for repair, it was never resolved until they gave me a replacement and it's been trouble-free ever since.