How to trouble shoot a sony sa-wm20 that stopped working

With subs, it's either

1. a blown fuse (no power at all at the sub);
2. a broken input (#1 cable, #2 sub input socket, #3 blown sub plate amp);
3. incorrect connection at the receiver end or the wrong settings

You've ruled out most of this because another sub works in its place. So what's left is either sub settings, or a busted sub.

The sub settings for testing should be: Crossover frequency - Max; Volume - Mid; Phase - Normal.

As a quick test, unplug the sub cab from the amplifier/receiver end. Switch on the sub and turn the volume down all the way. Briefly put your finger across to short the RCA/phono tip and ring. The sub should make a buzzing sound. You now know that the sub is working and the cable appears to be okay or not.

Most subs auto sense a signal. So if the Sony isn't doing that then there's the problem. Given that these are very much a budget sub, I'd be inclined to have a look for something simple such as a broken solder joint or bulging capacitor. If it's anything more indepth then the cost to repair will outweigh the value of a replacement.