So, long story short, I re-encoded one of the 96 KHz / 24 bit tracks to a 44.1 Khz, 16 Bit Mp3 (192 KBpS), and couldn't hear the difference either on my sennheiser earbuds or my car stereo (an alpine head unit with audyssey multiEQ signal processor and JL and Rockford Fosgate amps, so it's a fairly good reference as far as car stereos are concerned). I Haven't tried the difference with my Sennheiser HD 280's yet, but I don't know if I'm going to bother because there really wasn't any difference and I don't anticipate hearing any difference on the better headphones. They both sound great though; if you're looking for some reference tracks to compare audio quality between different settings on the One M8 or between different devices, I'd recommend setting up an account with HDtracks.com just to download the free sample album.
I think we're likely to see much better improvements in audio quality in other areas; better headphones / amplifiers & speakers, properly ripped audio files, and even equalizers such as boomsound or clarifi (I put those last for a reason but I'll stay out of the boomsound sucks vs. it sounds good to me discussion). Support for external DACs is a very, very exciting IMO; the One M8 should really shine with an externally powered DAC that is powerful enough to properly drive my HD280's or produce an real line-level signal for the aux input in my car stereo.