What is the best way to hook up a Denon DVD-2910 ?

Some DVD-A discs have only a multi-channel mix. The Beatles Love is one such disc. If he was to play one of these with the other channels active he would only be hearing the front two channels of the 5.1 mix. Instead of turning the other channels off in the players menu, just set the player to downmix multi-channel DVD-A discs. Also obviously for both DVD-A and SACD he will not be able to use a coaxial digital out. He will have to use the left/right analogue outputs. Most universal players,such as the Denon, have duplicate pairs of analogue outs for the main channels. One just on their own, and one set that are part of the 5.1 analogue multi-channel outputs. I too have noticed that on some players the left/right front RCA outputs of the 5.1 hookup will sound better than the stand alone left/right stereo RCA outputs. Additionally I have found that often with DVD-A discs, even ones with discrete hi-rez two channel mixes,that using the player in downmix mode to achieve stereo will sound better than the discrete two channel mix. I have no idea why. Since you are adding the Denon player to your system, just simply try both hookup schemes, and see which one you find sounds best.