So far I have been having rather "good" experience with 4-gen Haswell NUC (aside from graphics-related issues regarding Linux drivers).
My question is, what can be done to improve analog audio quality on Linux? Since there is no proprietary drivers for the Intel audio for Linux. Maybe buying a high-quality DAC would do?
Are there any type of audio cards that can be installed internally rather than external such as USB audio devices?
Quite honestly, analog audio quality on Linux + NUC, not a very good idea!
I you want to improve SNR I would think that you either use a HDMI audio converter or some USB audio device.
There are various DACs out there but I have no experience from their compatability with Linux (mainly Ubuntu).