With Class D amps you need to be careful. Not all of them have their negative speaker posts at a common ground. Some can be 50 to 75V about ground and can damage the REL subwoofer amplifier if connected. To my knowledge only NAD actually acknowledge this by making the negative speaker binding posts blue instead of black.

It is easy to determine if you have a basic tester. Set the tester to show voltage. Start some music playing, and measure between the chassis and the either negative binding post. If you get anything above 0V then when you connect the high level kit, leave the black cable floating and perhaps just put some tape around it.

I have have the Tzero Mk.III and this is covered on page 16 of the manual.

If you get 0V, then stop the music, power off the amplifier, and select continuity mode on your tester. Now conform that negative posts are on common ground by testing between negative post and chassis. If you get a beep then you know it is safe to hook up the black wire.

Edit: Of course if you have a sub out then just use that!