Should hook I speaker level sub to center or sides

I suppose the best option would be to get another amplifier and use the sub line out.

Amen to that.

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 don't think that the center channel has much LF information. It's primarily dialog. I'd run them as subs to the main L/R speakers. That's assuming that you already have a sub for the LFE channel in your theater. In not, I'd get an amp to drive one or both of them off the LFE line level out from your AVR or processor.

I always have a quality sub connected in parallel with my center channel. But, two of those, wow? There's a huge amount of deep bass and impact from the center speaker most folks never hear.