I recently came into psoession of the MOTU with the intent of using it as a mixer/router in my nascent live rig (VirusC, MPC1k, Live, Reason). I already owned the RME converters, but those serve a difference role, and don't offer some of the features of the 828...
so, the shootout: RME ADI-8 DS vs. MOTU 828 mkII
outputs from the synth and sampler were multed (at a patchbay) and routed to the above converters. the signals were then piped digitally** to my Apogee PSX-100 for D/A conversion & monitoring. I just switched back & forth between the 2 digital inputs (same audio into the RMEs and 828mkII, respectively) to the PSX with the press of a button and monitored on my Mackie HR824s.
The difference is not as dramatic as I expected, in fact, it's pretty subtle. I distinctly prefer the RMEs, the bass is slightly rounder and fully feeling, and the top end is smoother and less digital/plastic. Transients felt a wee bit smoother, too.
But I must confess, I'm far more impressed by the 828mk2 than I expected to be. It's A/D conversion is totally usable, and based on my first day impressions, the price to performance ratio is stellar. The CueMix software is useful for simple routing & monitoring, which is basically what I wanted...
HOWEVER...
When I did the corresponding D/A comparison, the RMEs *smoked* the 828mkII. Ate it for lunch. Night and Day. The RME D/As compare very favorably to the Apogee PSX to my ears. The Apogees have a bit more depth and dimension, and are clearly brighter/cleaner. The RMEs add some coloration, but it's not undesirable to my tastes. Both sound good...
otoh, the 828mkII D/As sound shallow and thin by comparison. The bass is discernibly muddier and less defined, and the "dimension" of the sound is greatly lessened. The transient response in the D/A stage seems greatly inferior, and plasticky artifacts are pretty plainly apparent.
I don't think the 828mkII D/As are unusable, but I do think that they are decidedly inferior to the RME ADI-8 DS D/As. (The A/D conversion, as noted above, is more closely matched.) For live performance, I'm pretty certain the 828mkII D/A will be serviceable --it won't affect the sound as much as a bad stylus would a record, or the amp or room will effect the sound, but I can state outright that I wouldn't settle for them for the purposes of studio mixing...
otoh-- you get what you pay for. $700 vs. $1400 (vs. $1700 for 2ch Apogees).
Also worth noting: The spces for the RME ADI-8 DS are different than those for the Multi/Digiface, I believe, so this may have limited applicability wrt the 828mkII vs. Multiface debate. Consult your dealer for details...