You may want to check in with the folks that frequent the Digital Audio forum here on MR.
There are a couple of ways to get good sound out of your Mac... one is using the optical cables into an external DAC as mentioned above, but if your receiver/amp lacks a DAC with optical input, you may find the choices rather limited and/or expensive. Stand alone DAC pre-amps tend to be targeted at audiophiles and therefore pricey.
An affordable way to get good audio out of a Mac (or any computer really) is with a USB audio interface (with built in DAC). There are tons of options from $100-$1000 with decent ones going for as little as $200-$250.
Apogee is a well respected company that makes these kinds of audio interfaces that target the Mac market exclusively. Their
Apogee One for the Mac is perfect for quality stereo output even if you won't make much use of the input capabilities.