I'm using early '09 iMac and latest OS. Only time I've every had issues if I was doing way too many things at once. Like ripping a movie, having Aperture open, and playing Audirvana + at the same time. But if I'm just surfing the web at the same time, I have no issues at all.
I use integrated mode and it's pretty seamless. Just like using iTunes. Only way I can tell it's Audirvana + is I have progress bar in iTunes disabled with one of the options. I found leaving it working sometimes caused glitches if was looking at iTunes store at same time as Audirvana + was running. With it disabled, no issues at all. This toggle is in one of the preference tabs of Audirvana +.