For the purposes of this discussion, I am playing guitar through an Axe-Fx II into a Behringer UMC1820 USB rack soundcard. OS is Xubuntu Focal 20.04.
This works fine for general jamming, I can play a backing track from the computer and use the hardware monitoring to mix in the Axe-Fx as necessary. But when I want to record, the levels are low. I record using my Android phone, and activate Audacity to record the guitar tracks from the Axe-Fx. But the levels are so low, the waveform is tiny. I can apply a plugin to increase the volume, but you can imagine the quality.
I see the same thing in the Pulseaudio Volume Control Input Devices tab, so it's not an Audacity issue. With jackd active, the input levels are fine, I can see them in Ardour. But I don't want to have to activate jack for more simple stuff like this, it's a pain.
For what it's worth, the output level of the Axe-Fx is around 75%, and the input gain knobs on the soundcard at about 25%. Any higher and the clip lights come on when I play hard.
Any clues on how to debug / fix this? I'm suspecting a driver issue, because there simply aren't that many moving parts.