How do I stop my subwoofer from humming?

The problem could be that the BLACK wire of the high level cable is not mated with the correct connection.  This wire needs to be connected to a signal ground in your system.  Typically this is the negative (-) output terminal of the amplifier, BUT NOT ALWAYS.  When you are using a balanced differential or Class D designed amplifier, the negative output terminal is never signal ground.

Signal ground can be found in a few places on most integrated amps, pre-amps, or processors.

1. RCA rings are frequently a source of signal ground.  Remember that RCA connections are coaxial.  The signal runs through the center wire and pin, and ground runs through the shield which is the outer  ring.
2. Phono ground lugs are an excellent connection to ground.
3. Antenna Earth ground connections are also a good connection

Solutions:

• Instead of connecting the BLACK wire of the high level cable, simply make a connection between an unused RCA on your system and the LFE input on the REL. The outer rings of the RCA will connect the REL to signal ground of the system.  Set the LFE level on the REL to minimum to avoid interference and tape off the BLACK wire of the high level cable to insulate against accidental shorting.

If you are connecting to a theatre system, you are already making this connection between the SUB OUT of your processor to the LFE input of the REL.  In this case just leave the BLACK wire disconnected and tape off to avoid accidental shorting.

• Solder the BLACK wire of the REL high level cable to the ring connection of an RCA plug and connect it to an unused RCA input on your system.
• Connect the BLACK wire to the ground lug provided for phono input on integrated amp or pre-amp.
• Connect the BLACK wire to the Earth connection for radio antenna on amplifier.

Radio antenna inputs are often compression or coaxial. 

For compression connections, simply push the release in and stick the black wire into the ground input.  For coaxial connections, the black wire can be wrapped around the threads and twisted together.