that's interesting about the RME having a bit-feature... i was shopping for one a few weeks ago and it seemed it was on backorder everywhere, so i got the D50s as a placeholder.
i haven't yet found out how to capture the digital output stream, but i did do a quick test using the analog output from the dac as input to my pc, recorded a few seconds of a song with audacity, then compared to the flac by aligning the waveforms, inverting, and merging... most of the result was close to null except near some sharp transients. i exported the data to csv and used excel to find that there was a temporal shift of 4 samples over the 2 seconds of data and when i manually adjusted for it by removing 4 individual samples from one curve, they overlayed very nicely.
seemed like the bit slips occurred just prior to some sharp transients.
i found it interesting, but can't say if it was due to the dac, my sound card, or audacity... might record the whole song to see which curve is longer
will keep looking for a way to capture the digital stream...