External subwoofer (asus) not working

Hello, I just installed ubuntu 16.04 on my asus N550JK which comes with a nice exteal subwoofer. Unfortunatelu, audio is not working very well, and I never managed to get this work in the past year on the past versions of ubuntu.
If I select 2.1 audio, the left and right are correctly tested, but when I click test for the subwoofer I hear a hiss/crackle sound. So it is recognized in some way, but not correctly.
Does someone know how to fix this?
Thanks

......:~$ lspci | grep Audio
00:03.0 Audio device: Intel Corporation Xeon E3-1200 v3/4th Gen Core Processor HD Audio Controller (rev 06)
00:1b.0 Audio device: Intel Corporation 8 Series/C220 Series Chipset High Definition Audio Controller (rev 05)

Schermata del 2016-07-19 09-50-23.jpg