Subwoofer hums when no audio source is connected
Although it is only an educated guess, my guess would be that you are picking up interference from your power lines and/or power supply. If the hum is low, that is likely the cause. If you can verify that the hum has a frequency that is either the same as, or a multiple of the line frequency in your country, then it is almost certain that is the source.
Why are you picking up such interference? There are a number of causes, many of which can be solved by proper grounding. [In some cases, connecting a component to ground may increase hum, by creating ground loops that act like big induction coils, so adding a connection to ground is not always the solution].
One source of hum is from power supplies, especially if the device is old and or poorly made.
If grounding your device eliminates the hum, then even if you cannot pinpoint the source, you might want to just ground the device and be done with it.
You mention that the device is connected to the wall outlet with a two-prong plug. Is that because the wall outlet has only two prongs (i.e. it is not grounded) or because the device only came with two prongs?
If you are using a &quot;cheater&quot; adapter to plug a three-pronged cord into a 2 pronged outlet, then the device probably needs a ground to work properly (without hum), and you should look into the possibility of updating your wiring to include grounding. I understand that in some countries, wiring buildings without grounding is general practice. In the US it is not, and only older buildings lack grounded outlets.