Been testing recent builds of Audirvana and Decibel. Without doing extensive testing, I was enjoying Audirvana since several RAM playback changes. Unfortunately, they now handle sample rates kinda differently. "Upsampling" on/off in Decibel adjusts based OS setting (off leaves as OS), Audirvana on track (I believe this has changed over time - have asked developer, but not heard back). So OS at 44.1 with track 96 in off positions will product 44.1 output in Decibel and 96 in Audirvana. Audirvana gives nice 'only 2x/4x' and 'maximum upsampling' options (Decibel just 'for best quality'), but effectively eliminates the downsampling capabilities - not so great for those of us with 16/20-bit DACs. So ideally, I'd be using Audirvana (which also happens to be the free option), but not with how sample rates are currently adjusted (no 'true' off). For those with 24-bit DACs I'd consider Audirvana.
Never tried Fidelia.
EDIT: Just got notified of an Audirvana update. Will retest the above (have to move DACs around), but beautiful new interface.
DOUBLE EDIT: Downloaded Fedelia. Audirvana still has the missing downsample functionality (if you consider it a missing feature - may be a plus depending on what it avoids in CoreAudio). Fidelia joins Decibel in allowing OS X midi control.