Oh, this is an interesting trade-off.
In the PulseAudio desktop scenario, we automute the speaker, and
PulseAudio remembers the individual headphone and speaker volumes. So in this case, there is no benefit from having individual headphone and speaker volume at the ALSA level.
However if a user wants to turn off automute, then there is a need for
being able to adjust headphone and speaker volume individually.
But it's not just a question of volume control for 2.1. Being able to
send a different stream to the subwoofer could be useful too, especially if the hardware filter is bad or non-existing.
It is not just low pass filter with subwoofer
Pulseaudio seem also perform high pass filter at the same time
http://cgit.freedesktop.org/pulseaudio/pulseaudio/commit/?id=f3ebf6b667b155f...
92hd91 have high pass filter for internal speakers
2.20.BTL Amplifier High-Pass Filter
For mobile applications, speakers are often incapable of reproducing low frequency audio and unable to handle the maximum output power of the BTL amplifier. A high-pass filter is implemented in the BTL output path to reduce the amount of low frequency energy reaching speakers attached to the BTL amplifier. This can prevent speaker failure.
2.20.1.Filter Description
The high-pass filter is derived from the common biquadratic filter and provides a 12dB/octave roll-off. The filter may be programmed for a -3dB response at: 100Hz, 200Hz, 300Hz, 400Hz, 500Hz, 750Hz, 1KHz, or 2KHz. The high pass filter is enabled by default with a cut-off frequency of 300Hz. The filter may be bypassed using the associated verb (processing state verb).