I always use Canare GS-6 instrument cables because they coil very easily and are 21 AWG. I discovered this week that the black insulator (under the shield) is conductive and should be stripped! It can short the cable if it comes in contact with the hot lead, and can also help the cable act as a capacitor, which rolls off high end. Man, I had no idea about that damn insulator, because when I made my cables ~6 years ago I didn’t RTFM.
I re-did them (some I was able to just strip down the insulation), and I don’t hear any difference in sound, but that doesn’t mean it shouldn’t be stripped away from the core conductor. Here’s what’s Canare is saying about it: