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Join Date: Mar 2004
Location: St. Hubert, Quebec, Canada
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If you want to ensure your iTunes to use the TX-8050's DAC, simply run an optical cable from your Mac Mini's audio output (you'll need a mini-toslink adaptor for the Mac Mini end of things) to your Onkyo. That way, your Onkyo will do the work.
The way you are currently connected means the Onkyo's DAC may be in use or it may not--it depends upon whether you engage DIRECT or PURE AUDIO on the Onkyo. If you use either one, then your Mac Mini's DAC is doing the work. If you do not use either of those settings, then the Onkyo is converting the analogue signal coming from your Mac Mini to digital and back to analogue at the final stage.
If you want to avoid such extra conversions, then use an optical cable like described above. Otherwise, you can leave things as is.