There is currently a bug in the Apple Core Audio driver that will prevent the Scarlett 18i20 from streaming audio with the device set to 96kHz. This happens because the channel format doesn't change for the higher sample rate.
To resolve this please go to Applications > Utilities > Audio/Midi Setup. In there please select the 18i20 and change the channel format from "18ch-24 bit interger" to "14ch-24 bit integer" on the Input tab. Then change the channel format from "20ch-24 bit integer" to "16ch-24 bit integer" on the output tab. Please do this when the computer first starts, before streaming any audio or opening any audio software.
A note to Logic & Garageband Users
After changing the above options, Logic and Garageband can display the following behaviour:
1) When starting Logic, regardless of what samplerate is selected in Audio Midi Setup and the DAWs project, Logic will cause Audio Midi Setup to switch back to the higher channel settings (that we changed in the above article).
2) If you are starting a 96khz session, or wish to work in 96khz, with Logic still open, please open Audio Midi Setup, and reset the channel numbers to 14 & 16 as above article. This should mean you're now able to record and playback at 96khz in Logic. NB-THIS CHANGE NEEDS TO BE SET BEFORE STREAMING ANY AUDIO WITHIN LOGIC.
3) Garageband also displays the same behaviour as step 1), in that it auto-changes channels back to 18 & 20 when opening, the difference here is that Garageband is currently hardwired to 44.1khz, so no issues with working at 96khz. If you then close Garageband, you will notice that the Audio Midi Setup stays the same, and so if you wish to listen to playback in OSX at 96khz, you will have no audio. Once again, navigate back to Audio Midi Setup and change channels back to 14 & 16 as original article above.
If you have any further questions please contact us here.