Orb super 8 subwoofer was working fine for almost 2 yrs,suddenly began emitting loud hum.quits when I turn off the unit(on-off switch on rear)or by unplugging the unit. The first subwoofer was defective after one yr,so this is my second from orb audio.I'm disappointed in the reliability of these units.

Filter capacitors in the amplifier or a broken ground (shield) wire. Check if there is a ground lift switch in your system and try it in both on and lift positions.

It sounds like the problem is not with your VR-606. If you hear a hum from the sub, the problem is more likely to be in the sub's built-in power amplifier. However, it is worth resetting the amplifier to factory presets. To do this, unplug the power cable from the mains supply and reconnect whilst holding down the power on/standby key. If the hum persists, connect any powered speaker to the sub-out socket, power on and listen for the hum. If no hum then suspect the powered sub amp. Turn the sub on with no connection to the Kenwood VR-606. If hum persists, the sub has a problem and will need to be replaced or repaired. By the way, you said there is a low frequency hum through the 'speakers' for about 30 mins. Is that all speakers or just the sub? Hope this helps.
Cheers, Don.

1.Check the input cable or change it and see the result.
2.Test the subwoofer to a computer.

Disconnect the input and see if it hums in the absence of an audio source. A bad audio cable shield or unwisely-routed audio cables will allow entrance of unwanted signals from external power sources, magnetic fields, even dimmer-controlled track lights. Sometimes, simply reversing the orientation of the ac power plug can eliminate humming.

try taking out the capacitor and wire the amp up with out it. it sounds like the cap is back feeding or discharging voltage when it should not be . if you reinstall the amp with out the cap and noise goes away buy a new cap or do away with it .

you have a bad colour wheel

There is a problem with the amplifier within the sub. Many of these units never completely turn off. They are often set to automatically startup when a signal is detected. This may be the case here. The loud hum as a defect in the output amp section. At this point you probably have a blown amp section. The "protection" for this is to cause a dead short that will open up the fuse. This unit will require service. It is unlikely that any service info is availabel to the general public either.

The fix could be simple, but without seeing the unit, I can not state that for sure.

Keep us posted.
Dan

There should be an 8 pin IC close to the input jack that will be marked 4558 or 4560. This is an Op-amp Ic that is the most likely cause of the problem. Replace that IC and report the results here.
Dan

The circuit that should be turning the sub on and off based upon the presence of an audio signal is not working correctly and the sub is trying to reproduce 60 cycle hum. The unit needs service.
Dan

sounds like a fuse is getting overheated and tripped or to loud, check your volume settings again.