Harman Kardon TS-7 Subwoofer - Wont come out of standby - Assistance to troubleshoot (have schematics) : r/AskElectronics
Basically, all Harman Kardon amplifiers (including subwoofers) show power supply issues after a few years « The Wiert Corner – irregular stream of stuff
This is how you reset your Harman Kardon Citation speaker - Coolblue - anything for a smile