Sennheiser GSX 1000 Audio Amplifier for PC and Mac
Sennheiser GSX 1200 PRO Audio Amplifier for PC and Mac

See Sennheiser - Gaming Downloads for firmware updates for the GSX series under Firmware Update Tool under Surround Dongle Downloads.

TechPowerUp - Sennheiser GSX 1000 Audio Amplifier Review by Inle, on Feb 7th, 2018.

YouTube - This Headphone Amp will Blow Gamers' Minds!!! by HardwareCanucks published on 2016-12-02.
YouTube - 7.1 Surround Sound - HEAR IT YOURSELF! by HardwareCanucks published on 2016-12-07.

TechPowerUp Forums - Reviews - Sennheiser GSX 1000 Audio Amplifier

Tested to work on Slackware64 14.2.

To enable the GSX to work with PulseAudio do the following:

1. Use the following command to get the card number assigned to the GSX:

   aplay -l | grep GSX

2. Example output:

   card 3: Audio [GSX 1200 Pro Main Audio], device 0: USB Audio [USB Audio]
   card 3: Audio [GSX 1200 Pro Main Audio], device 1: USB Audio [USB Audio #1]

   In this case the number is (card) 3.

3. Archive the current default.pa:

   cp -p /etc/pulse/default.pa{,.original}

4. Open /etc/default.pa in a text editor.
5. Find the lines:

    ### Automatically load driver modules depending on the hardware available
    .ifexists module-udev-detect.so

6. Add the following lines BEFORE those lines:

   # capture device, card 4, subdevice 0.
   load-module module-alsa-source device=hw:3,0
   # playback device, card 4, subdevice 1, 7.1 channels.
   load-module module-alsa-sink device=hw:3,1 channels=8

   Change the 3 in hw:3,0 and hw:3,1 to the card number shown in the output of the aplay -l shown earlier.

With this setup using the volume dial on the GSX will result in audio being muted instead of adjusting volume, so leave the dial alone.

Source: GitHubGist - augustocdias/sennheiser.md

Tested to work on Slackware64 14.2 (with latest patches as of 2018-11-02).

This assumes the steps described on SlackDocs - howtos » multimedia » pulseaudio_non-default were followed to disable and *not* remove PulseAudio.

The following ~/.asoundrc content works to enable a GSX 1200 Pro as the primary audio device with 7.1 (8-channel) audio and direct mixing (dmix) so multiple applications can play sound on the GSX. It also results in a working volume dial on the GSX.

pcm.!default {
	type plug
	slave.pcm "dmixer"
}

pcm.dmixer  {
 	type dmix
 	ipc_key 1024
 	slave {
		pcm "gsx1200pro"
		period_time 0
		period_size 1024
		buffer_size 4096
		rate 44100
	}
}

pcm.gsx1200pro {
	type hw
	card Audio
	device 1
	channels 8
}

ctl.dmixer {
	type hw
	card 1
}

Based on an example taken from the Software Mixing dmix example on AlsaProject - Asoundrc.

This also produces correct output for speaker-test.

Speaker-test command:

speaker-test -c 8 -t wav -l 1

Output:

speaker-test 1.1.1

Playback device is default
Stream parameters are 48000Hz, S16_LE, 8 channels
WAV file(s)
Rate set to 48000Hz (requested 48000Hz)
Buffer size range from 2229 to 4458
Period size range from 1114 to 1115
Using max buffer size 4456
Periods = 4
was set period_size = 1114
was set buffer_size = 4458
 0 - Front Left
 6 - Front Left Center
 2 - Front Center
 7 - Front Right Center
 1 - Front Right
 5 - Side Right
 4 - Side Left
 3 - LFE
Time per period = 11.318586

The following .asoundrc content works to make the GSX 1200 Pro the default audio device, but speaker-test will only play the sound for Front Left and Front Right speakers, speaker 2-7 are shown as Unused.

defaults.pcm.!card Audio
defaults.pcm.!device 1
defaults.pcm.!ctl Audio
defaults.pcm.!channels 8

Based on an example taken from Gentoo Wiki - ALSA - Configuration - ~/.asoundrc.