Problems with my Chiro c 800 and 5.1 decoder

Howdy, I picked up a Chiro c 800 and a 5.1 decoder pretty cheap on Craigslist. I'm running it to a 5 channel ATI amp through PSB Imagine B bookshelves in the front and some cheaper Energy speakers in the rear. I have the decoder hooked up to the preamp as well. The problem I'm having is I'm not getting any sound in the prologic mode, only in stereo and enhanced stereo. It's faint and I can only hear some sound when it's turned up all the way My question is if there is definitely something wrong with the unit or if possible there is a connection problem. Perhaps the cable I got for the decoder is the wrong one. Anyhow, I really like how the Chiro sounds in stereo mode and since I listen to a lot of music on it, this is important. Would it be worth it to fix the unit or put my money towards something modern? If anyone has a copy of the manual that would be a huge help too! Thanks for your help.