There might be something wrong with the one you have and/or its just not truly compatible with the Mac.
It's not a common setup so options are probably limited…perhaps limited to some higher-end interfaces (that have WAY more features than you need) if you want reliability.
I suppose you've considered going into the speakers analog either directly from the Mac or via a USB to analog device, if possible.
Is the digital to analog converter on the speakers better than the one in the Mac or third-party adapters?