Uh, are you sure that you can use those channels as a passive sub-woofer out-put? I've always thought that surround channels had highpass filters on them. If this is the case, that is why you cannot get any bass out of them - because there is no bass going to them.

 

In some new receivers the Pioneer VSX-D912 that I have an I beleive Sony does the same thing. You connect the positive wire to the right channel positive connection and the negative wire to the left channel negative connection. Then in the receivers menu you select surround back speaker setting to subwoofer.

 

With the older Dolby pro Logic surround, the rears did not recieve a full-range signal (and it was mono to!). Now, dolby digital(5.1), Dolby Prologic II, DTS (and a myriad of other DSP signals) send a full range, stereo signal to the suround channels

 

That's odd, when I hooked up one of my old Kenwood 12's to my receiver it moved the sub quite a bit. The sub was 4 ohms and the receiver was 8 ohms, but surprisingly it worked but would cut out when you turned the volume up. (It was an old receiver) It was giving the sub about 100 watts. Maybe something is wrong with your sub.