I just recently got a Mac computer and so am trying out Audirvana. I found the instrument separation to be so intense that I felt the kind of fatigue you can get with too much stereo separation on headphones. After a bit of searching, I found a super simple crossfeed audio unit component that you simply put in the library/audio/plugins/component folder. It has just a few settings after you activate it in the Audirvana preferences for feed level and for low pass frequency. It is based on the Boris Mikhaylov's Bauer stereophonic-to-binaural DSP library.
With the crossfeed turned all of the way up, it makes the soundstage much more natural and cohesive. It truly is a relief as it takes the edge off of the hyper-real separation of the Audirvana software by itself.