Thank you r/diyaudio. Your advice to change out the capacitors got my friend's subwoofer working again with no loud humming noise. I can't wait to see his reaction when he gets to
Troubleshooting the Internal Amplifier of a Subwoofer – Klipsch
Why is my speaker making strange noises? | B L Acoustics Limited
Sunfire HRS-8 subwoofer hum and pop : r/audiorepair
How to Fix Subwoofer Hum with No Input | BoomSpeaker
How to find and fix hum in 3 easy steps – PS Audio
SOLVED: Loud humming noise from subwoofer - Fixya
The Five Cs of Subwoofer Setup | Wirecutter
What Can I Check If My Subwoofer Is Humming – Arendal Sound
How to Stop Subwoofer Hum That is Caused by a TV Set Top Box | REL