[Jackaudio] Distorted sound out of JACK-consumed programs

[Roberto Carraro]

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