To take Todd's comment a little further.

Which of the components has the better DACs, the DVD or the ES receiver?Since they're both Sony, probably little difference,but more importantly, does the receiver pass the 5.1 inputs in analog form to the amp section?
If it does then you can swap connections and see if there's a difference. But if it doesn't,then I would suggest using the digital input. That way you're not converting the signal twice.