I am having a very similar issue with an amplifi fx100 to a crate stereo combo amp. I was running from the amp out directly to the stereo-in the back of my combo amp so there's no effects on the amp running. It was working fine for 3 months until a few days ago the signal got a lot less "hot" and the effects just suck. I tried swapping guitars, swapping cables, moving the amp and pedal to different areas of the house (to mitigate electric interference since I just set up a lot of electronics near where I usually play) to no avail. The only way to get the good sound now is to use the Mono(left) output, although I'm still plugging it into the stereo-input on the back of the amp. It's not the amp, the cable, or the guitar. Is the amp-out jack failing? Like I said it used to be a good connection and now it's weak.