Harman Kardon Onyx Studio 2 Troubleshooting - iFixit
PARTS REPAIR Clear Harman Kardon SoundSticks II Subwoofer Stereo Speaker System 28292501814 | eBay
Did you have the Harman Kardon subwoofer upgrade? : r/Challenger
Basically, all Harman Kardon amplifiers (including subwoofers) show power supply issues after a few years | The Wiert Corner - irregular stream of stuff