TL;DR: It appears the iPhone 6 supports 24-bit audio and 48 kHz playback.
So I found a way to empirically test playback.
This first screenshot shows a recording of playback over iPhone USB into Quicktime. You can see from the spectrogram that there is data in the 22-24k range, which means that playback is at least 48 kHz.
This next screenshot shows playback over USB straight into audacity. Basically, I played -95 dB and -97 dB tones at both 16- and 24-bit audio. -97dB is below the minimum volume of 16-bit audio. While all the other tones made a sound, the 16-bit, -97 dB chunk of the recording shows no sound, while 24-bit -97dB does play a sound.