Connect Subwoofer to Audioserver or stereo extension

[Nick Everard]

Hi Guys 

Has anyone had any success connecting a subwoofer to a Audioserver or stereo extension?

I have an REL T5 subwoofer with a neutrik connector, connecting in the standard format yellow / red to left / right channels and the black to black speaker out the sub just produced a loud hum. Following some research I understand the Stereo extension has a class D amplifier connecting the ground cable to the stereo extension 0v input I can stop the hum but to even with the sub level at maximum the volume is minimal. 

Any ideas? Or am I being blind and missing something obvious?