iRoc vs. Musicstream: I've had the opportunity to listen extensively to both. The iRoc is a special DAC. If you need a modestly priced DAC, get an iRoc and call it "Done!". It doesn't merely do very good digital, it does exceptional music.
If you need something that rivals or exceeds the virtues of a top flight analog front end, get a Musicstream. It's absolutely superb and not a small step up from the iRoc!
Shazam, wished you could have heard the Musicstream in the context of our room/system at RMAF. Sonically, we weren't trying to spotlight any particular piece of gear but rather present a musical system where you could appreciate the virtues of a given piece of gear in regards to its contribution in a system. In that context, I believe the Musicstream shined and, while I wouldn't say we had the "Best" room at the show (I don't think anyone had a "best" by what I heard...), we did have one of the more enjoyable rooms evidenced by quite a few folks who came back more than twice for an extended listen. This was no small thanks to Ultra Fi's electronics contribution.
Short of it: Depending on what you're willing to spend, you'll do very very well with either.
Disclaimer: I have no business interest in Ultra Fi. Mr. Moore and I have simply become fans admirable of each other's work.
Cheers!
Robert
RSAD