Hi,
The sub is driven either from pro-logic amp that is capable of simulating an LFE (low frequency effect) sound channel from the pro-logic sound, or by a dolby digital amp that correctly feeds the .1 in a 5.1 sound track to the sub.
If you only have a nicam stereo TV, there is no sensible way to get a sub feed, unless you take the Left / right audio to a pro-logic / dolby digital external amp.
email me if you want to know more details.