You have a number of options to experiment with.
I've tried the headphone jack, RCA, and M-L (using a BS 3200) with no sound quality loss to BL 4000, and BL 2000 linked speakers. A mini-splitter plug works very well.
DVD: I used to watch DVD via the PC, configured via: PCDVD ->PLASMA ->BEOPORT -> BS 3200. I would have to find a N.RADIO non-broadcast station ( such as the Goverment stations which are clear most of the time ) to listen to the DVD sound.
Now using HDMI for sound and picture, with 1 cable only into the Plasma and back to the BS 3200 and Beoport.