Re: KZ PR2, US Apple dongle and USB Audio Player Pro.
Since we've been getting a few reports of this combination being sufficient to drive the PR2 to an adequate SPL, I thought I'd try it for myself even though UAPP doesn't fulfill my requirements in an audio player.
It does work and did drive the PR2 to an enjoyable volume. If I were to guess, I'd say ~63 dB maximum. I can't precisely as I've not run a meter on it - I might attempt to later. However, since I normally listen to my speaker system at approximately that level, it was close in comparison to my ears.
I should qualify this statement by saying I'm perceiving as ~63dB. My meter actually showed the PR2 it reaching 80 dB.
I, personally, like a bit more volume than that, and I prefer more powerful amplification to handle transients.
Anyway, those were my findings regarding the situation. Your milage may vary.
Happy listening...