UPDATE: this is out of date and although it worked for me at the time, there is a better solution to make it work: https://github.com/evilphish/sennheiser-gsx-1000
In order to make the Sennheiser amplifier GSX 1000/1200 work on Linux as a 7.1 device it is needed to change a configuration file of pulseaudio. Execute this command to identify the output sound cards:
aplay -l
This is a sample output
card 1: Audio [GSX 1000 Main Audio], device 0: USB Audio [USB Audio]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: Audio [GSX 1000 Main Audio], device 1: USB Audio [USB Audio #1]
Subdevices: 0/1
Subdevice #0: subdevice #0
To identify the input sound cards:
arecord -l
This is a sample outuput
card 1: Audio [GSX 1000 Main Audio], device 0: USB Audio [USB Audio]
Subdevices: 0/1
Subdevice #0: subdevice #0
For the playback, subdevice 1/1 is the 7.1 playback device ("Speakers" equivalent on Windows) and the subdevice 0/1 should be the "Headset earphone". Edit /etc/pulse/default.pa, uncomment the lines below and set the correct device:
load-module module-alsa-source device=hw:1,0 # capture device, card 1, subdevice 0
load-module module-alsa-sink device=hw:1,1 channels=8 # playback device, card 1, subdevice 1, 7.1 channels
Information taken from here
Remove the # before load and remove also everything after the other # include himself
exemple :