Found this on iFi’s site.
“What we have done is disable the USB device volume control (which uses DSP in the XMOS system to control volume). OSX “obeys” this while other operating systems have additional DSP (usually optimised for low CPU load over sound quality) that adds extra volume controls that are preferably left alone. So we have not “locked away” anything from Mac users, instead, OSX avoids an extra layer of DSP (as does Windows in exclusive mode WASAPI). If volume/balance is adjusted this is in all these operating systems, it invariably causes loss of quality and it is not recommended to use these functions. Most speakers and headphones have sufficient channel matching that balance adjustment is not required.”