A ball valve on the bk is happy making. Like groundchuck, I use a 90deg elbow down into the flat-bottom pot and it leaves only 8 oz behind. 'O course, that low it picks up a good amount of crud. Tradeoffs.
I got 2-piece valve for economy. If you can, get the 3-piece. I pulled apart both in the store and IMO the 3-piece cleans more surely.