There should be a guitar center at seven corners and another further out west near George Mason. I didn't see any Grados there but I saw some AKG, AT, and Sennheiser models. I don't think I saw any of the higher end models like the HD600, K701 or the AD700 if that's what you're looking for.
There is a store in Kensington, MD called SoundWorks that carry Grados and maybe some others. The Myer Emco chain carries Sennheiser and the F.Y.E. chain carry Denon and AKG. All of the above won't have all models in-store to listen to, so I would call ahead