What is the correct subwoofer connection??

I have a Von Schweikert VS-1 powered sub. In the directions it says to hook up to my pre amp. I have a Bel Canto pre3. Hooked up to it is a digital/ss amp and 2 cary slam 100 monoblocks. All outputs are taken by these amps. So I have hooked up the Sub to each channel on the Digital/.ss amp. I was thinking about getting a 2nd sub because the room is 24 x 24 with obstacles. Should the sub(S)be hooked directly to the pre amp? If so how? Thank You
You could run a splitter off of your bass amp outputs to send signal to sub no?