Mac OSX does not support native DSD. It only supports DoP (DSD over PCM).  Since encoding DSD into DoP requires twice the bandwidth, when using NuPrime's DAC with Mac OSX, the support of DSD is limited to half of the maximum decoding capability of the DAC.
From Audirvana:
“To listen to DSD files from a DAC, Audirvana Plus transmits the DSD files to the DAC using the “DSD over PCM” protocol that requires markers, and using this protocol requires twice the bandwidth over the native DSD signal only. In order to get the best possible result, you need to use Audirvana Plus with a Native DSD DAC. However, if your DAC is not DSD compatible, Audirvana Plus will perform downsampling to PCM using a very high quality 64 bit algorithm.
Please note : The “DSD over PCM” protocol requires markers to identify the signal as DSD, so using this protocol requires double the bandwidth compared to the native DSD signal alone. This can reduce the maximum DSD frequency that can be used with DAC by half compared to the frequency that can be reached under Windows. This may explain why a DSD 512 DAC can stop at DSD 256, and DSD256 DAC can stop at DSD128.”
“DoP is a method to encapsulate DSD native signal in PCM stream. There is no conversion to PCM. It is true DSD signal, just that OSX thinks it is PCM. The DAC recognises it correctly and decodes the DSD signal. So DSD over PCM 1.0 is the correct setting”.