Comment 25 for bug 1274401

the major problem is how pulseaudio know when the headphone or speaker share the same volume control if the driver create "Bass Speaker Volume"

in your case the subwoofer is connected to the mono pin which downmix stereo to mono
the driver just copy front channel to both DAC instead of using 4 channels output

some asus notebook can also connect to sonic master external subwoofer

https://bugs.launchpad.net/ubuntu/+source/alsa-driver/+bug/1040873

according to the user comment. the subwoofer seem only use right channels of the stereo complex
in this case they need pulseaudio to mix left and right to provide signal to the external subwoofer

http://lists.freedesktop.org/archives/pulseaudio-discuss/2013-December/019654.html

e.g. a desktop with 3 audio jack at rear panel and 6 channels codec (e.g. alc662)

the rear line out and front panel headphone have to share the "Front Playback Volume" control since multi channel to support 5.1 is more important than the "headphone playback volume"

but a desktop with 6 audio jacks at rear panel and 10 channels codec

rear line out jack use Front Playback Volume" exclusively because there are enough volume controls for each output jacks and headphone use "headphone playback volume"

there are different case for a 4 channels codec

e.g. those thinkpad with dock station are also have three output ( headphone, dock headphone and internal speaker)