Does a sub cable diffferentl than an interconnect?

No, been there done that.

The same. Some IC's do bass batter than others.

"Need to have amps widely spaced for decor reasons. Prefer to buy one high quality 20" rca connector (i.e., a sub cable) instead of two cables (standard interconnects). Will this make a sonic difference? Would use the same brand for the longer and the shorter lengths (Audioquest)."

If you are connecting a full range speaker, it may very well make a difference. Also, there's absolutely nothing that would suggest that a subwoofer IC would be any higher quality than a regular IC. You would have to look at it on a case by case basis.