Did you ever figure this out? I bought an HX effects last week and I'm having the same problems. My amp is only putting out about half, at best, of what it's capable of. All cable connected as per 4cm instructions. Mix of the fx loop is at 100%, return at 0.0dB. If I unplug everything from the amp and go straight in with the guitar it's as I expect it to be, very loud.
Pretty frustrating.