Here's my situation.
I have two JL amps. 500/1 is powering a subwoofer and it's working perfectly.
JL 300/4, however, does not show green light for power. I checked voltage at the amp and it measures approx. 12V. 300/4 is powering two front comp system and it sounds fine. I mean they get loud without distortion. Could it be just the LED bulb on the amp went bad?
Thanks in advance.

 

Simple way to tell if the amp is not working is turn down or off if possible, the sub volume and lower the gain on the 4 channel. If the speaker's sound lowers then the amp is functioning properly. Make sure you remember or mark where it was originally. You don't want to go higher (or much higher) than what it was at before. Sending a higher signal to the speakers could destroy them (more power to them).

 

LED's do go bad sometimes.