Work backwards from your amp; with it on, plug in a cable(only), touch the bare plug. Getting 60 cycle hum/ buzzing? Good. Then plug the bare end into the looper output, and then a cable into the input(with bare end). Touch that, get noise, good, and then repeat that over and over with all the pedals until you reach the beginning of your pedal chain. If there are any of your boxes with level controls, turn those up(you can get back to unity gain later after you’ve done your troubleshooting). Are all your connector cables absolutely working? All of your pedals powered correctly? One indispensable tool to have is a cable tester, usually not much bigger than a stomp box, which will accommodate a variety of plugs/connectors, and give you continuity/short status. During the insanity of failing gear at a gig, while it may not fix the situation, it can help calm frazzled nerves a bit in that you know what’s going on with your lines. Are you using the amp’s effects loop in any capacity, or is it a straight path from bass, thru pedals, to front end of amp?
