Good advise as a rule of thumb.Going more specifically, a lot has to do with the age and dielectric... and if it was exposed to the elements. PVC wire from the 50s and 60s that's remained indoors is going to be slightly dry/brittle by now, and it can't handle much heat. Cotton/wax sleeved tinned copper is actually desired by a lot of people refurbing amps as it has a particular sound. It's still flexible, but doubtfully up to code/UL listed for power applications. Older vintage PTFE / Teflon wire that's decades old is still perfectly good from what I've seen. Polyethylene, although introduced to the market in the 50s, I haven't stumbled upon (or looked for that matter), may fall somewhere in-between. Fortunately with RCA interconnects, etc., it doesn't have to be rated at 300 or 600v like a power cable would, so experimentation is safe.... nothing will be catching fire ...try and round up some cotton/wax tinned copper wire (not the new stuff) and see what you think.