Hi

I found the manual for your JBL sub. Looks like a nice piece of gear.
I copied the following section from Troubleshooting portion of JBL manual:
If there is no output from
the subwoofer, check the
following:
• System is on and a source
is playing.
• The HT PS300 is plugged in
and “Power” is on.
• If you are using the
THX input, make sure the
THX/ Normal switch is set
to “THX.”
• If you are not using the
THX input, make sure the
THX/ Normal switch is set
to “Normal.”
• If you are using the “High
Level Inputs,” check all wires
and connections between the
receiver/amplifier and the
subwoofer. Make sure all wires
are connected and that none
of the speaker wires are
frayed, cut, or punctured.
• Adjust the “Output Level”
control.
• Review proper operation of
your receiver/amplifier.
_______________
All of that is pretty standard troubleshooting stuff.
You might try adjusting/increasing the Sub Output Level on your receiver.
Are you using an AVR?
If yes, which make?
I use a Harman AVR154. Some people on this and other forums told me that Harman AVRs have a weaker Sub Output?
Not sure if that is true.
This is just an example of how Sub Amps can react differently when hooked up to different signal sources, i.e. Pre-amps, AVRS, studio gear.
I have 2 MCM 200W Plate Sub amps as indicated in previous posts. I also have an O-Audio BASH 500W Sub Amp.
Neither of the Sub amp models I have would work with their built in Auto On circuits when hooked up to the AVR154. I tried turning up the Sub Output Level on the AVR with no improvement

In a small home studio, I use a consumer/pro type Tascam TM-D1000 digital mixer hooked up to a stereo power amp and one of the MCM 200W sub amps.
When hooked up to the Tascam mixer, the Auto On circuit of ALL the sub amps works properly???
Yes.
No kidding.
Hook any of them up to the AVR and none of them will turn on with the Auto On circuit!
______
If you can find a JBL service manual for your sub and isolate the resistors that control the voltage on the Auto On section, you might be able to get it to turn on and stay on the way I did with the MCM sub amps.
Some of the other guys on the forum here are really great at finding fixes for these kind of problems.
I wish I could be more help.
Have a great weekend and good luck with the sub.
It looks like it will be really nice once you get it going
