Ok, so check this out...
.I have everything connected: FX Send from HD400 to input on the front of the amp, guitar connected to Guitar In on HD400, Effects Loop button on the HD400 is on. I went into settings, and the Effects Send level is at 0db, and increasing that increases the volume of the amp model (as it should). If I disable the amp model, I have absolutely no sound at all, and it doesn't matter what effects I assign to FX1, 2, and/or 3, or the level of any of those 3 FX options. It's as if disabling the amp model on the HD400 just kills the sound all together on this thing, which is what I can't figure out.