Check the link below for an A/B test of four AD/DA converters for Pro Tools HD: 192 I/O, HD I/O, Apogee Symphony and Prism ADA-8XR.
The test was conducter in Metropolis Studios Studio A by world-class engineers as a real double-blind test, no sales guy blahblah or blue LEDs blinking on your face. All inputs and outputs were calibrated to 0.1dB accuracy and same clock source was used for all I/Os for practical reasons. Check out the video, you might be surprised. The most expensive is not automatically the best. We will conduct a similar test soon, stay tuned.