I bought the ART USBmix but can't get the configuration with my guitar right. Problem is that the interface seems to send the guitar (from guitarix or other..) back and creates a feedback loop. I tried everything that I can think of, but I am not an expert on Linux audio, ... far from that. Fiddled around with Jack, Pulseaudio, etc. but all I get is feedback or no sound at all.
Can you explain how you have set up your USBmix?
I use Ubunty Studio on a HP EliteBook 2560p with my headphones on the USBmix, guitar in CH1, USB playback assign button pressed.
Kind regards, Bas