Long story short, if you need use an Avid S3L-X with macOS and playback via AVB, do not install macOS High Sierra. macOS Mojave works fine, as does the older macOS Sierra, but High Sierra has clocking issues that manifest as constant clicking during playback, rendering the audio unusable.
If all you need to do is record via AVB, macOS High Sierra works without issue.
References
- http://duc.avid.com/showthread.php?t=396987 (I’m sfsetse in this thread.)
- http://duc.avid.com/showthread.php?t=392402 (older)
- http://duc.avid.com/showthread.php?t=393255 (older)
Note, all versions of macOS High Sierra through 10.13.6 are affected.
[Update 2019-03-03] I continue to have no problems with macOS Mojave (currently 10.14.3).
[Update 2018-09-29] Preliminary testing with macOS Mojave (10.14.0) and 64-channel recording and playback indicates that the AVB problems have been fixed.
[Update 2018-07-28] Increased affected versions to 10.13.6.
[Update 2018-06-05] Increased affected versions to 10.13.5.
[Update 2018-03-31] Increased affected versions to 10.13.4.