DSP (digital) = 5 band eq.. also there is a DSP button somewhere by the faceplate. There is also a nokia subwoofer in the trunk.
nonDSP (analog) = tremble / bass control only.
Honestly since you don't even know which one you have, you are better off doing some extensive research on E39 forums for sub/amp installs instead of looking for what "sub" to get. But here is a quick summary of the CONS.
DSP = requires a LOC & Factory amp has a built in bandpass filter for 35-45Hz (no way around it). Which means it doesn't really matter what sub you get.
non-DSP = requires balanced input amplifiers.