Rear panel of Denon receiver
The important thing about the back panel, and this picture, is that there are four optical (digital) inputs, which are compatible with the digital output of the Mac. So when you play an MP3 from the Mac, and connect to the receiver with the optical cable, the D-to-A conversion is done by the receiver. The Mac is $500 of computer hardware (and damn good at what it does) and the Denon is $2K of audio hardware, and also very good. This setup lets each system do what it does best. The result is stunning sound. Really hard to explain how good it is.