Comment 26 for bug 344588

alsa seem only implement the capture devices only

do you have additional capture device (hw:0,2) ?

arecord -l

Take a look at block diagram of alc889 datasheet and the codec-info section in the output of alsa-info.sh

Ten DAC channels support 16/20/24-bit PCM format for 7.1 sound playback, plus 2 channels of
concurrent independent stereo sound output (multiple streaming) through the front panel output

The "6stack" or "6stack-dig" model connected both HP and Line Out (Green) to the front channel [Audio Output].

you need to implement a new model "6stack-dig-fronthp" which connect your alc889's alt_dac_nid 0x25 to HP out at Ext Front and modify the driver to create stream_analog_alt_playback (device 2) and create the volume control/switches

0x02, 0x03, 0x04, 0x05 [Audio Output] connected to those four [Pin complex] Line out at Ext Rear ( Color = green, black, orange ,grey )

0x25 [Audio Output] - connect to [Pin complex] HP out at ext front