I'd be surprised if you could tell the difference between a high-bitrate mp3 and the "original" WAV. Combining that with the fact that folks claim that the empeg has to keep the drives spun up all the time when playing WAVs, I think you definitely ought to go the mp3 route.

Seriously, just take an hour, rip a complex track (preferably one that has a wide range of sounds, bass, treble, quiet, loud, etc.) and encode it a number of times with different settings. You can even try WMA if you want to, but I'd be wary of it for non-technical reasons. I'm sure that you'll find a limit at which higher-bitrate encodings sound no better. And I bet it's no higher than about 200kbps.