Alsa-info patched kernel:

http://www.alsa-project.org/db/?f=cd23a3f0bb72e38d73224620e3e8ab57a73aa441

Chmap results:

Type = FIXED, Channels = 2 FL FR Type = FIXED, Channels = 4 FL FR NA LFE

When unplugging the subwoofer, then chmap gives this and stays like this.

Cannot open PCM stream hw:1 for PLAYBACK

[Pavucontrol] -> [Configuration] -> [Built-in Audio] gives

Analog Stereo Duplex Analog Stereo Output Analog Surround 2.1 Output + Analog Stereo Input Analog Surround 2.1 Output Analog Surround 4.0 Output + Analog Stereo Input Analog Surround 4.0 Output Analog Stereo Input Off

Seem pulseaudio bug since your laptop does not have surround40 chmap

It is not clear how pulseaudio's 21 channel-map match with alsa's 2.1 chmap

http://cgit.freedesktop.org/pulseaudio/pulseaudio/tree/src/modules/alsa/mixe...

[Mapping analog-surround-21] device-strings = surround21:%f channel-map = front-left,front-right,lfe paths-output = analog-output analog-output-lineout analog-output-speaker priority = 8 direction = output

[Mapping analog-surround-40] device-strings = surround40:%f channel-map = front-left,front-right,rear-left,rear-right paths-output = analog-output analog-output-lineout analog-output-speaker priority = 7 direction = output

Pulseaudio verbose log while plugging in the subwoofer:

http://pastebin.com/Ya5u1qz6

https://bugs.freedesktop.org/enter_bug.cgi?product=PulseAudio