Maybe there are a couple of tings you can try first.
1 Connect an MP3 player or similar into one of the stereo inputs (e.g. 19/20) on the mixer, and play material with a good bass component in it.
2 Hook up any powered speaker to the mono out and see if you get anything. Leave the low pass filter off.
3 If not, try another lead.
4 Connect sub to one of the main outputs and see if anything happens.
5 If nothing is happening, try connecting the mp3 player directly to the sub
Um . . . check for really obvious things, like is the sub on? Is its volume up?