Sparky, to summarize some of what John and Randy have said, if you're playing a DVD you'd use Dolby Digital or DTS 5.1, depending on what's available on the DVD. Playing a regular CD, you wouldn't be able to use DPLIIx until you got the speakers for 7.1, since what DPLIIx does is to take 2 or 5 channel material and expand it to the back 6th and 7th speakers. With a 5.1 speaker setup, you should consider making full use of them on CDs and other 2 channel material by using DPLII Music, which takes the ambience naturally present to varying degrees in 2 channel material(no artificial reverberation or anything else is added)and steers it to the surround speakers where it belongs.It also steers sound equally strong in the left and right channels to the center speaker, forming a real rather than a "phantom" center image. You should experiment with the center width and dimension controls on DPLII to get the most pleasing results with various source materials.
