A few things to consider:
1. On digital cable, not all channels are digital. You are only using analog cables so this doesnt really matter, but on my cable, the analog channels are much lower in amplitude than the digital channels.
2. Converting a two channel analog feed to 5.1 via Pro-Logic is a matrix operation; ie it derives the 5.1 channels from 2 (unless it is dolby encoded, in which case there is *some* info as to what sounds go to what channel). Not all material will sound good when converted by the Pro-Logic decoder, particularly shows with mostly dialogue like sitcoms and sports. Zipper's suggestion to use an all-channel stereo mode is a good one - or even use plain stereo for those channels.
3. Make sure you at least make a valiant attempt to calibrate all channels equally using an spl meter and a calibration disc, like AVIA or DVE. Tweak to taste from there; eg increase the center level to make voices more clear.
4. If you have the opportunity to get a cable box with digital out and your receiver has digital in, it is well worth it. Most channels on my digital cable that are actually digital
broadcast in Dolby Digital 2.0. The DD 2.0 signal can be converted to 5.1, 6.1, etc by Pro-Logic II decoder.