Second your impression on Audirvana Plus. Not only it sounds more natural (particularly when compared with Amarra / Amarra HiFi) , the layering, imaging and sound stage are excellent as you can sense the location/tone of individual instrument in an orchestra. Further more with direct play and integer mode turned on, unnecessary software layers (for various audio effects and sample rate conversion) such as Apple Core Audio are bypassed all together, thus settings in Apple MIDI Setup utility do not have any effect on Audirvana Plus. For pure music appreciation by audiophiles, any additional audio processing has adverse effect on sound quality.
Audirvana Plus requires certain amount of memory (buffer size is adjustable) for track preload and processing, scheme for sample rate conversion is adjustable as well (according to capability of your external DAC). It doe not modify any setting in Apple MIDI Setup at all (see above), thus no audio setting should be affected once Audirvana Plus application has been quit (does not matter it ran in hog mod / direct play / integer mode). So if you never set these parameters properly according to your Mac and DAC hardwares and the application keep crashing and audio system never work properly for other applications, then blame yourself instead of the application. With Audirvana Plus, crappy software such as iTunes and all unnecessary software layers can be bypassed completely, thus it should be more stable than Amarra HiFi and Pure Music and even standalone Amarra.
By the way we are talking about paid commercial software Audirvana Plus but not free open source software Audirvana. The free version has some critical components (particularly good sample rate converter) missing.
I have also been trying out audirvana plus and I'm likin it a lot. It does make an improvement from just using iTunes, at least to my ears.
I was wondering if you are doing any up sampling also. I was playing around with none vs max and I think there is a slight difference between the two (source is 16/44.1 ALAC). What are your impressions on that?