DACs:Apple device vs receiver

[copernico]

I got a Yamaha RX-A710 receiver and I use the iPod touch (2nd gen) to stream radio to the receiver via an airport express unit. Right now the AE is connected to the receiver with a regular mini plug cable to one of the RCA audio inputs. I am considering using optical cable from the AE unit to a digital input in the receiver. As far as I understand, this would be better if the DAC in the receiver is better than the DAC in the airport express. I also have the option of plugging the iPod directly to the receiver, and in this case the iPod would be doing the D to A conversion. Are the apple devices generally good at DACing compared to the DACs built in a receiver?

cheers and thanks