I had a similar experience with an Amplifi 150, just sound out of the tweeters no woofer. Here was my diagnosis path, big thanks to @rdownings for a good starting point. So I unscrewed the rear panel, inspected power pcb and woofer amp pcb couldn’t see failed components, so removed the power supply board at the bottom, put the connectors back on and ran it outside the case and checked and had all the voltage rails , +-45v +-19 and +8v, same issue. Was going to swap out the woofer amp board as I have 2 150’s but decided to check all the audio signal cables and connectors and it started working, so might have been un seated connector or could still be dry joint while knows, but it’s working. But yeah think checking voltages off power pcb before removing it is the fastest thing, then audio connectors , and then amp module. I got lucky !