if you only use one channel, then you are getting half the signal. a sub signal is a mono signal. meaning that there is no left and right channel (however sub amps do have left and right inputs) so lets take ac/dc back in black for example. just about everyone has heard the intro to their song. notice the sounds from the left and right? how an instrument starts one one side and then the other instruments come in? well imagine that with an instrument that is supposed to have a little bass behind it and you only got sub signal from the left channel. this means that every instrument on the right channel will not have any bass. the amp has a left and a right channel input, but the signal coming out and going to the woofer is from both channels. meaning that the sub will play both left and right bass. imagine that the drummer is on the left side of stage but you are only getting signal from the right side, well you would have officially tuned out the drummer at this point. so in short, it is always necessary to get both left and right signal for a sub in order for it to sound correctly