Having problem - ADA 890

Hello !
I bought recently Altec lansing ADA 890 speakers and i Have some problems with them.
i have connected the rear and front analog cables and also the digital cable to my Audigy 2 soung card.
There are different modes in these speakers ( stereo, stereoX2, prologic, quad, Dolby Digital).

I have two main problems
- When I play songs or watching movies with my computer In Dolby Digital mode, I barely hear sound from the rear spekers.
but when I changed the mode to quad mode, I heard good sound.
When I play songs or watching movies with my computer In Dolby Digital mode, sometimes there isn't sound at all from the rear speakers, and when I changed to another mode it sounded good.

- When I am doing a channel test with the program that came with the sound card (audigy 2) there isn't sound from the rear speakers with Dolby digital mode and when i changed the mode to quad mode there was a sound when I did the channel test with the audigy 2 program .

someone with Audigy 2 card and ADA 890 speakers can tell me what should I do ???

thank u very much
R.e.L