I have a tube integrated amp and a Polk PSW505 sub. I've connected the left amp O/P to the left main speaker & the right amp O/P to the right main speaker.
I've connected the left pre-amp O/P to the left Line In of the sub, and the right pre-amp O/P to the right Line In of the sub.
I get no sound out of the sub, except a low level rumble.