Hello Chesseroo,
Hope you are well. The only way I can see of hooking multiple subs to every channel, or even two subs, would be to set all speakers to large and use the high level inputs to the subs, and high level outputs to the speakers.
However this would not be optimal due to added distortion of the double amplification of the signal by the subs amplifier. Of course the question is, would the distortion be audible or objectionable, probably not. The benefit, most probably, would outweigh any downside of trying this.
Later,
TonyM
