connecting DD decoder to a 2ch preamp

I have a Dolby Digital decoder built into my DVD player. What's the best/easiest/cheapest way to connect this to a regular 2 channel preamp without 5.1 inputs? right now, I can connect the Left and Right main outs from the DVD player to my preamp, which works fine for the main front channels, but what about the other outputs (Center, L rear, R rear and Sub)? The DVD player's decoder doesn't adjust the volume/attenuation of the DD output, so do I need a 4 channel preamp for the different levels for the 2 rear speakers and the subwoofer? should I just buy a cheap add-on DD decoder in order to be able to adjust the levels of all 5.1 outputs needed for DD?