UPDATE: this is out of date and although it worked for me at the time, there is a better solution to make it work: https://github.com/evilphish/sennheiser-gsx-1000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