[Jackaudio] Distorted sound out of JACK-consumed programs

Hello, I'm new to audio production in Linux, so apologies in advance if I will use wrong terms and bad jargon. I'd like to record my guitars and bass, but I've found a problem which I think it can be JACK-related. I bought 2 USB audio interfaces, a 2-channel Behringer which I returned and a Focusrite Solo 3rd gen, which I'm testing right now. The issue that I have is with the programs that use JACK, like Guitarix and Reaper. It seems I can only get distorted sounds out of them and both have JACK as input source. Instead, Audacity, which lets me choose the hardware audio interface directly, lets me hear the sound of my instrument just fine. What I've tried so far: - my everyday Ubuntu 18.04 with low-latency patch: Guitarix and Reaper distorted audio and Audacity just fine - live AVlinux: Guitarix distorted audio and Audacity just fine - live Ubuntu Studio: Guitarix distorted audio and Audacity just fine Can someone please help? What else can I try? I'd like to stick with Ubuntu 18.04, as it is my main everyday desktop. Thank you. Roberto