I don't have an answer why you're seeing different results when trying to improve downstream vs upstream, but I can suggest a solution that might eliimate the problem.
Do you have coax feeding your router? Do you also have coax feeding your TV downstairs?
If so, what I'd suggest you do is use a MoCA adapter downstairs to provide a wired Internet connection there. I'm using an Actiontec ECB2500 to accomblish the same thing.
http://www.actiontec.com/251.html
Should you still need better wifi coverage downstairs, once you have the MoCA adapater in place you could use its output to feed a second wifi adapter, or router. That would eliminate the problem of wifi signals having to penetrate walls and floors from your upstairs to your downstairs location.