J_evege,
The DIN connector on the back of the Opus 21 is not attached to the variable volume control inside of the Opus 21. I've read several accounts that suggest the DIN output sounds a bit better than the variable RCA output.
Since you use a pre-amp, you could use the DIN output from the Opus 21. You need to acquire a cable that is DIN on one end and dual RCA on the other end. I think Resolution Audio recommends the DNM cable, but they have a relationanship with DNM, so you need to take that into consideration. I know other cable vendors will build a DIN to RCA cable for you.
Some other component manufacturers use the DIN connector, so there are other cables on the market intended for a mixed DIN/RCA setup. Just be sure they are wired correctly for the RA Opus 21.
Enjoy,
TIC