DVD Audio-Why can't I get my disc to play in 5.1?

I recently purchased a Cambridge Audio Azur 540d dvd player. Although the unit is in my opinion superior to many players I have found it frustrating in the extreme,to use.
The most recent example.I purchased the dvd-audio disc Tommy (The WHO) just to see what one sounded like. The problem is I can only get it play in stereo. My receiver a Denon 1603 automatically goes to stereo and displays 96k direct on its screen. no amount of fiddling with either the dvd players menu or the receivers,will get it to play in 5.1.
For the record the Cambridge is supposed to play dvd audio discs in full surround.
The receiver is a Denon 1603.
speakers-Tannoy Mxr all around.
Center Channel-Tannoy Mxr CC.
Sub-Mirage FRx 8
The system plays movies in dolby digital or DTS with no problems.
The only hang up appears to be the damn dvd audio playing in stereo only.
Any ideas or thoughts as to why this should be or how to correct it would be most appreciated.