So i've got a randall RX120Rh and i'm trying to run a GNX3 effects processor through it and an EHX Metal muff. I'm mostly going to use the GNX for delay reverb chorus etc.. However i still like some of the patches on the GNX. So i ran the GNx trough the FX loop and for some reason, now only some of the patches are really loud and just generally sound horrible. Also i have to strum really hard to get any sound out at all. I'm also getting sound when the volume on the amp is completely off. I can't seem to figure out the problem.
Try running it in front of the amp to see if you still have the volume issue.

Maybe some of the patches have the volume/gain turned way down?

That kind of pedal with a metal muff wouldn't sound that good anyway me thinks.

I think Digitech has a patch and forum section on their web site, you could try there.