There are a number of options for playing music on a Mac into a stereo:
- Apple TV
This would work if he has an A/V receiver with an HDMI input. Paul has an older receiver with RCA inputs though, so he'll need something else.
- Apple Airport Express
This is $99, and actually is a wireless router with an analog audio jack. He can tell iTunes to play out to the Airport Express. This wouldn't be much different than taking the line out from the computer, though.
Since Paul has high-end audio gear, he probably will be interested in something called a DAC (or Digital to Analog Converter). A computer's soundcard can convert digital to analog, but it could be noisy and not as high-quality. There are products that will do this conversion outside of the computer:
These are just a couple of examples, but there are a wide range of choices anywhere from $15 to $15,000.