I assume that by "source" you mean an amp/dac combo. In that case the MP-302 reviewed by Skylab seems to do well with the Denons. Also I would read up on something like the Audio GD Fun, Sparrow or NFB11 or 12. Maybe something like the JV5, NuForce uDAC. There are so many option at that price point, and many good ones.
If you looking for just an amp, there are even more options. I would highly recommend the Schiit Asgard, the Headroom Micro amp, or something used like a RSA XP-7.