I agree with most here who suggest that your USB interface was the likely culprit. I'm not familiar with the DAC you were using but IME most DACs with multiple input options have a severely compromised USB input. If you are dead-set on USB you should look at a standalone asynchronous USB DAC. This is what I am using at it beats the pants off of my previous CDPs - including SACD/DVDa on more expensive machines. In fact I used to have the very same Sony player you do and my Mac Mini --> DAC is so much better it is laughable.
Another thing to watch out for is to make sure your software player is transmitting a bit perfect signal to the DAC. Itunes alone is a no-go, you need aftermarket add-on software.
Another thing to watch out for is to make sure your software player is transmitting a bit perfect signal to the DAC. Itunes alone is a no-go, you need aftermarket add-on software.