I have had three (cheap) subwoofer cables fail in the last year. Definitely wiggle the cable or try a different, known good cable.

I'm not sure why cables act up more with the sub, but I suspect it has something to do with the wire not hanging down like it does when connected to electronics... so a bad connector can show up when you use a cable for subwoofing. This is just a guess, of course.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8