I would definately use a good quality RCA.
I have a Pioneer CD Player and a Yamaha AX-759SE and the RCA provides a nice, warm clean sound with clear treble etc ......
When I try a good quality Optical (only option on this setup) it instead gives a clinical, emotion less performance. My opinion is that the DAC's in an AV amp are designed for the range needed for vocal/effect/surround purposes and no matter how much you spend on an AV amp that will always be the case.
Obviously the more you spend on the AV amp, the more you will spend on a CD player and so in theory you're CD will always have a better DAC than the amp. The only way you'll better it is to go for a high quality external DAC but hey, if you were doing that you'd have a dedicated HIFI set up along side your AV one wouldn't you?