You have two choices:
1. Get an optical cable and connect it to your Onkyo. This will pass the digital signal straight to the amp without any sound processing being done by OS X and will allow you to send a 24-bit, 96kHz signal.
2. Get a USB/Firewire DAC with pre-outs, such as the audioengine D1 http://audioengineusa.com/Store/D1-24-Bit-DAC which will process the audio and send it out to your amplifier.
Regardless, if I were you, I'd play my music through something like Audirvana+ to get the most out of it. It bypasses coreaudio completely and uses iZotope instead, which is a much better sound processing engine.
http://audirvana.com