Really stupid question - Are you selecting USB as the input on the 818.3? Also have you checked in the programming that USB is actually mapped to the USB input?

The 818.3 won't automatically select the source if something is plugged into the USB input.

Your Mac is seeing the 818.3, so you're over half-way there!

Good luck,

Steve

Main: 861v6 (LPS), 808v6, UHD722, DSP8000SE Anniversaries, DSP5200.2HC, DSP7200.2, SW1600, Sky Q, Cambridge Audio BD, Roon, M80
Office: 861v4 (LPS), 818v3, 800v4, HD621, M&K S150P, DSP5500HC, M&K MPS2810 sub x2
Son's system: 861v4, HD621, DSP6000.2, DSP5200HC, DSP33
Spare: MCD, 100 Series, 200 Series, 500 Series, F80s, C15s.