I've spent some of today analysing this issue. It was a little sad that nobody picked up Andrew's patch upstream, but writing model parsers are deprecated today.
Anyway, I believe there is a simpler solution, so can you please try to edit /etc/modprobe.d/alsa-base.conf and add this line:
options snd-hda-intel model=asus-mode4
...reboot to test. If this does not work, follow the instructions here: https://wiki.ubuntu.com/Audio/UpgradingAlsa/DKMS
Now try these new drivers in combination with the above configuration file change.
Also note that you might need to raise the "Bass Speaker" volume in alsamixer as well.
If any of these approaches help you (and does not cause any other regressions), please respond with your alsa-info ( https://wiki.ubuntu.com/Audio/AlsaInfo ) so I can send your fix upstream. Thank you!