The Yak Kayak stacker rotate down if you have Yak bars. Otherwise you have to take them off.
I agree with the Thule's square bars are more stable than Yakima's rounded bars. But I don't like the bulk of Thule's kayak stacker, they take up way too much room on the rack with my small car.
So I have the Thule bars (square) and the Yakima Stacker posts. I can get 4 boats on my little tiny car, or two boats and a Thule box.
Yakima makes most of their items convertible so they work on either square or round bars.
You ARE a soul, you HAVE a body.