I've tried it both ways and there is plenty of bass in both the center and sides. Now the question is which is more accurate. No idea since this is new to me and I have no reference. Next I'll try one hooked to the center and the other to one of the sides assuming both sides get similar bass.
I suppose the best option would be to get another amplifier and use the sub line out.
I suppose the best option would be to get another amplifier and use the sub line out.