I have a JBL SUBWOOFER MODELPSW-1200 that hums as soon as it is plugged into the wall plug, there are no speaker wires connected to the subwoofer!

Try the following steps to stop subwoofer hum.

1. If the subwoofer is plugged into its own outlet, plug the subwoofer's power cord into an outlet shared by the other components. Use an extension cord if necessary.
2. If the hum persists, and your system includes a cable TV, disconnect the cable line. If the hum stops, install an isolation transformer on the cable.
3. If the hum persists, disconnect the remaining component cables one at a time until the hum stops. Install a coaxial isolation transformer on that cable.
4. If the hum persists, install a line-level ground loop isolator on the subwoofer's line-level feed cable. This usually solves most hum problems.
5. If nothing helps, disconnect all the audio cables from the subwoofer. Leave the subwoofer plugged in and switched on. If the subwoofer hums, it points to a defective unit. Contact the company's customer service to technical support.

Hum usually indicates a power supply problem. If it has an internal supply you will need someone to check it's producing clean DC. Alternatively if you do open it (beware risk of shock!!) initially visually check condition of /replace any bulging electrolytic capacitors.

check your connections, RCA, Speaker wires, maybe there is grounded. try other wires which you think in good condition..

Hit the search engines "speaker hum"

For example...

http://www.axiomaudio.com/tips_eliminate_hum.html

Are you using the Creative speaker test program? It usually has a picture of a generic layout of a pc system, with the speakers around it, showing a speaker for each that you should have plugged in. There should be a test button that will play a sound through each speaker. If a sound plays from your center speaker when the subwoofer picture is lit up then you may have a cord plugged in incorrectly on either the subwoofer or on the back of the soundcard. I've ran into this many times.
The best way to set up surround sound is to get your speakers placed or mounted on the wall, then run one wire at a time into the subwoofer, to ensure that the front left is hooked into the subwoofer in the front left, front right into front right, etc. Then once the speakers are hooked into the subwoofer start running the outputs from the subwooofer one at a time into the sound card. After each connection run the test program and watch the pictures light up as the sound plays, and make sure that the speaker playing sound in your place is the same speaker that is lit up on the test program.
I hope that helps, otherwise the amp is probably bad. Let me know if you need more help.

Service manuals: http://manuals.harman.com/JBL/HOM/Service%20Manual/PB10%20sm.pdf

http://manuals.harman.com/JBL/HOM/Service%20Manual/PB12%20sm.pdf

Take some voltage reading according to the manuals.

Most humming sounds in speakers is due to electrical currents or wifi signals nearby the speakers.

Do you have any electrical cables near the speakers? Do you have any wifi device near the speakers?

If so move them out/away and see if there is any difference.

Sounds like you have a bad connection somewhere. Make sure all of your connections are secure. If theres anything that is a little loose you'll get that hum. You might also try replacing your cable too. If it became pinched somewhere it could cause that problem

-Captain Jake

If you have checked the fuses which it sounds like you have, your amp may be out. Take that RCA and plug it into the other speaker. If thar she blows, treat as warranty and call the factory. If this is helpful, please post as such.

If your wire connections are normal then the problem is due your woofer speaker. Its your subwoofer circuit error and must be repair thru replacing the damaged Component in the circuit