May be this 2016 answer of AR Customer Service would help you :
"Hello Eric,
Audio Research has determined that an updated USB board and firmware are needed to communicate with the USB port of the latest Apple computers and the latest operating system, El Capitan. For DAC8s that were under warranty as of September 30, 2015 (the release date of El Capitan), Audio Research is pleased to offer this update under warranty. The updated USB board and firmware are under development by engineering and we expect updates will be available starting early summer.
Audio Research has also identified an alternate solution. This is the M2TECH Hiface 2 USB to SP/DIF Interface. When inserted into the USB port of the Apple computer this product provides a RCA or Coaxial connection to the SP/DIF inputs of the DAC8. This link to the manufacturer’s website offers more detail:
http://www.m2tech.biz/en/hiface2.html. Extensive sound room testing has shown that the M2TECH provides sound quality commensurate with that of a direct USB connection.
If you prefer this solution and are an original owner, please contact Audio Research customer service at
service@audioresearch.com . We will ship one to you at no charge. Please note that acceptance of the alternate solution makes the DAC8 ineligible for a free installation of the hardware and firmware.
Again, thank you for your patience as Audio Research works through this difficult problem. I look forward to helping you.
Best Regards,"
I choose to connect my NAS Synology with a M2Tech Hiface and i'm very happy with it.