CoreAudio is capable of bit-perfect output up to 32-bit, with no resampling at any sample rate.
It also includes a lot of optional capabilities like sample rate conversion, audio processing units, etc. but JRiver doesn't use them.
Now that we do DoP bitstreaming on Mac, it's easy to test that we're bit-perfect because if any bits in the 24-bit DoP package are not correct, the signal collapses. DoP is working on several DACs we have here, so I think we're now on pretty solid footing with the JRiver audio engine on Mac.