I listened to both at length, and I'm talking several occasions and hours of critical listening. Unfortunately, not in the same system... But I know these two systems very well!
First, to get the most out of the Opus 21 you must use the fixed-level DIN output. It's much superior to the RCA, just no contest.
But even then, it's not at the same class as the CDSA. I heard the EMM seperates numerous times and the CDSA is very close. To nitpick, you could say that it lacks a tiny bit in bass detail and about the same about the upper harmonic content. It's by no mean lacking in extension at both frequency extremes and definitely not lacking in overall musicality and integration.
The Opus 21 with the DIN output is a great player for the price. At this level, you will not get any "night and day" differences and by itself each of these players is very good. If you compare them side by side then I have to give the vote to the CDSA.
If you watch boxing, sometimes you have two competitors which are both good. It makes for a boring contest (for lack of music, maybe?) but sometimes one has a tiny edge over the other and he wins most rounds. The final score looks like a shut-out and does not tell how competitive the match was. That's how I would put the Opus 21. It might loose on many points but it's a darn close match!
