If you want to connect your Airport Extreme directly to the PWD, you need the PS Audio Bridge, with connection via ethernet.
Edited by boatheelmusic - 8/9/14 at 7:04pm
I use a powerline adapter that supports airplay via a usb DAC. It comes with a generic one but I replaced it with my wolfson based USB DAC and it works beatifully.
I'm a paid spotify member and I use this to enjoy spotify on my hifi daily. sure works a treat