Connecting Tube Amp to Sub

tomacco

I have a tube integrated amp. 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.

I have connected the polarities correctly.

Any ideas?