Connect from Send of DT25 to guitar input of Pod, not Pod effects loop return. Then use send from Pod to return on amp. Make sure you have the FX Loop block in the Pod signal path, and make sure it's on.
Basically unscrew the back cover, gently pop the knobs off the front (all of them) with gentle pressure and a couple butter knives or screwdrivers, and then remove the standoff screws inside on the PCB. There are a couple ribbon cables you will need to disconnect to remove the boards. They are held in place by hot glue. If you slowly wiggle and pull you can get the glue off and disconnect the ribbon cables. Once all the necessary cables are disconnected and you pull the switches board out, you can pull out the main board. It's a matter of angle and even pulling to get it out. You will have to remove some nuts from the back to release the 1/4" jacks. Once everything is loose, pull it out, desolder and solder. I did it over a year ago, so I may have forgotten a few minor details, but just look everything over and nothing should require any forcing. It all comes apart and goes together with gentle pressure.
Each model is based on a channel. That is why there are multiple models of the same amp. There are 3 Soldano models to cover this, as well as a few amps having a "normal" and "bright" model. If you want to use the same amp model with different gains, you will have to set them up separately and "switch channels" between them.