I recommend the Audience Au24 interconnect. You can usually find a pair on Audiogon. The Opus 21 is not truly balanced, but you may get additional gain through the balanced outs. However, the RCA cable may work fine for you too for less money. There is a new review of the Au24 on 6moons.com right now. The Au24 is my favorite overall cable.
But, if you really want to get the most out of the Opus 21 use an Audience Au24 DIN-RCA cable to bypass the volume control. You won't be able to find these used since they are essentially custom cables but it sounds great! It's about $600 for a meter, more than I prefer to spend but I did anyway because I like the Opus 21 so much and love the Au24. I tried the DNM DIN-RCA cable but was underwhelmed by it.
I've found that power cords do make a big difference with the Opus 21. I'd recommend an Audience PowerChord (yes, Audience again; they are just a good value) or even better a TG Audio power cord. You can probably find both on Audiogon.
Finally, i recommend using a Neuance platform which makes another big difference. You can get a single platform and place both components of the Opus 21 on it, which is how I use it, or get two separate platforms for the ultimate isolation. The single platform would be cheaper.
Enjoy the Opus 21. It's great!