[RaspberryPi] Forwarding USB Audio Input to HifiBerry AMP2

For music playback at home I have built my own Bluetooth Speaker. It’s a RaspberryPi 3, on top a HifiBerry AMP2, and they are powering two B&W DM601.
I have used this script to setup bluetooth and configure the audio settings.
Because bluetooth is the only way to pass audio to the raspberry (besides running music on the raspberry itself, e.g. via MPD/MPC), I was thinking of a way passing audio via chinch cable. I grabbed an old USB Soundcard I had laying around and connected it to the raspberry. Now I had to forward the USB Soundcard Audio Input to the HifiBerry AMP2. I found a way using PulseAudio for this. Got it from here.

#find input device (my USB Soundcard)
pactl list sources
#find output device (my HifiBerry AMP2)
pactl list sinks
#set loopback
pactl load-module module-loopback source=src_name sink=sink_name

But most USB Soundcards only have Mono-Audio-In and often there a annoying noises through the input… So this was really no joy and I will continue using bluetooth.