ya analog is definitely better in yours and most cases.
1. dolby digital is compressing the audio stream so it can use only 1 cable to send the 5.1 sound. the compressed format is 640kpbs only
2. your logitech decoder will decode the signal and play back the audio using its own DACs which is expected to be way inferior to zxr.
so if can, stick to analog surround...although it will be a bit messy with 3 cables coming out instead of 1.
next thing is try to play a music cd on your aego m with zxr and compare to your integrated audio. sure gotch different. enjoy your new audio bliss siol!
oh if you going lhs, may want to consider buying better graded cables. not sure if your logitech can take regular rca cables, if can, get some belden made by lhs and the audio will be even fuller....but each these custom cable strands will be thicker than your average computer audio cables. cost i think $4 per meter and need 2 pairs of rca plugs per strand...so for one 2m strand will cost $12-14...hehee can bargain abit.