I have the same problem. don't as yet have a solution
While walking in my neighborhood, a JBL PB10 was thrown away for garbage pickup. I can use another subwoofer so I brought the orphan JBL PB10 home. It has the same problem. I am a retired electrical engineer. If I get the schematic, I might be able to fix it. I indeed found the service manual from the following URL. It turned out you can fix the problem by just cutting off one pin of a transistor. I don't think most people are interested in theory, but just a brief and then the solution.
JBL PB10 service manual http://akdatabase.org/AKview/albums/userpics/10004/JBL%20PB10%20Service.pdf
The manual is for the version 6.3 circuit board. I have a version 7.3. But, they are the same for my purposes. The LED lights are controlled by an integration circuit shown on page 28. When the signal at the low level line input is smaller than 5mv rms, the LED lights are set to red. In the meanwhile, the integration circuit set the Q1 junction FET transistor to on so the SIG signal is grounded to subdue any nuisance of noises (no sound produced). When there is a significant signal at the line input, the integration circuit set the LED light to green and turns off the Q1 transistor so the SIG signal can be freely amplified by the AIM power amplifier.
When the integration circuit malfunctions, most probably C42 fails (could also be C40 and/or C41). I don't have any capacitors at hand to fix so I just cut off one of the three pins in the Q1 transistor to disable it from grounding the SIG signal. Q1 location could be found on page 26. The pin to cut off is one of the two outer pins. It is the one further away from C40. Once you cut the pin off, the sound will be back. But the LED lights remain red.
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