Instead of using the amp's pre-amp, you're using the HX Stomp instead to shape your sound.
By plugging directly into the Effects Return, you are going straight into the Power Amp, so it is literally just the Master Volume (though you can control the level from the HX Stomp). I'd advise start with the HX Output set to about 9 o'clock, and turn the MV up from 0 til it's loud enough.
Running the POD Go into the Effects Return of an Ashdown MAG 300 combo, with the amp's MV at about 9 o'clock, and the PG output at around 9 o'clock, it's easily loud enough.