mullhead,
You can do a pretty simple A/B listening test b/w the analog and digital outputs of the 5900 into the receiver.
Setup both sets of cables and use the cd and dvd selection on the receiver as the switcher with the cd selector set for the analog input from the 5900 and the dvd selector as the digital input from the 5900.
You can play a disc and then switch quite fast b/w cd or dvd input with your remote. This way you can compare the sound as produced from the DAC of the 5900 or from the receiver.
However there is one caveat, volume levels. The units may not have an identical signal to gain level thereby making one unit sound louder than the other. This typically equates to a user preferring the louder component and thus concluding various things like "it has more punch, sounded more clear, more detail...[place adjective here]".
Unfortunately you cannot equalize for SPL in this simple A/B method but in the very least, you can measure it (the SPL of the analog and digital inputs) to see if the volume levels are higher with one connection over the other. If they are not, then a proper blind test can be done between the two components as long as you have a willing assistant.
