NTL digital does not transmit anything other than standard Stereo Sound on the main channels (some of the movie channels use 'surround sound' but not true DTS or 5.1). However, pumping you NTL Dig signal through that box will 'probably' improve the sound as it should try to process the signal and 'enhance' it. Playing DVD's on this should sound perfect as they will have DTS and the player will use all six speakers correctly.
The speakers are designed to be 'spread around', ideally front speakers either side of the TV, speech and subwoofer in front of TV and satellite speakers behind you. That way you get the full 'surround' effect. Try watching the Fellowship of the Ring on the new player - about half way through they are on a hill top when a flock of birds approaches - in 5.1 you can really hear the birds fly in from behind you on the right
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HTH