The woodiness of Northern Brewer is an excellent pairing with brown ales. NB and cascade work well together for Am Brown. That is what Pete's Wicked Ale used.
Janet's Brown uses Northern Brewer, Cascade and Centennial. It's a great example of an American Brown. If I were to create my own recipe these are the hops that I would use.
I bitter with magnum and add Cascade later in the boil. The last brown I brewed I just bittered with magnum to my desired IBU and that was it. Turned out pretty dang good.