??
Why you are not using the digital input from the 218?
Sorry that make no sense for me - is there a special reason why you do that?
You can use either SPDIF (electrical) or TOSLINK (optical), so you avoid the bad D/A conversation in your PC and avoid also the A/D conversion in the 218.
You can try to make this and it guess it should work without cracks to your Mocho.
The volume is done correct in the 3200 (over IP control and SL) and the volume control for the headphones is done in your Mocho to make it correct without loose resolution.
So just turn the volume down for the 3200 and they are quiet, and the Mocho only have to play without crackling noise when it get a valid digital signal, otherwise the problem is the Mocho "logic".
Please tell me your results.
800v3/218/271/8k.2/5k5s

/5kC/DSP3200/Reviver/
TT-ELP/Graham Slee Revelation/Trinnov Altitude 32/OPPO 205
__Ayon/HB/Shunyata/Miltzow -- power cables,
__Ayon/Jungson/Miltzow --analog, Miltzow-- digital,
__Vortex/WMA ,