I changed my Audio MIDI sample rate to 24/196 and AHD reports the correct device capability and appears to be playing back at the correct resolution without me having to change the sample rate or bit depth.
When I play HD (as opposed to UHD) tracks, AHD reports the correct sample rate once again.
Can I assume that the Apple audio renderer is not changing the sample rate or altering the signal in any way?
Not sure if you meant to type 24/192 or 24/96, but either way the Apple OS is going to convert everything to that resolution. So if the Apple MIDI output resolution is set to 24/192, then everything - 16/44.1, 24/48, 24/96, 24/88.2 - will be upsampled to 192kHz, and any 16-bit source will have 8 bits' worth of zeros padded onto each PCM word to convert it to 24 bit.
The padding of the bit depth is totally irrelevant - in fact, most "bit perfect" software that claims to leave the original signal totally unaltered will still pad 16-bit (and sometimes 24-bit) content with extra zeroes to match the maximum/default/internal PCM bit-depth of the receiving DAC.
So the sample rate is the only issue. This is the major weakness of Apple's built-in core audio processing: it cannot do on-the-fly resolution switching. I used to be very concerned about this, and as a result purchased the $10 BitPerfect app, which sits between iTunes/Music app and the underlying MacOS, allowing on-the-fly resolution-switching (and also integer mode rather than floating-point mode, which keeps things bit-perfect).
However, thanks to
@danadam and others, I have become a lot less concerned about sample-rate conversion since I became a member of this site. Even with non-integer conversion (like 44.1 to 96) it's not likely to produce any audible effect.
In fact, I keep meaning to turn off BitPerfect and do some listening to see if I can hear any difference even in a sighted/biased comparison test. I suspect I won't, but it will be an interesting experiment. (If I do hear a difference, even if it's just confirmation bias, I will keep BitPerfect in the chain, since it does no harm and the $10 I spent on it was spent long ago and was totally worth the peace of mind it afforded when I was under the impression that it mattered more than it actually does. Sometimes practicality outweighs the effort of trying to fight one's own confirmation bias
. )
Personally I feel like upsampling philosophically just makes more sense since it's adding more data rather than removing it (though as a practical matter it doesn't matter). So I would recommend setting the Audio MIDI output to 24/96 and then forgetting about it. You could set it to 24/192 I suppose, but I think 192kHz (and 176.4kHz) sample rates are so unnecessary and dumb that I personally would choose 96kHz myself.