I was just grading papers while playing back some recordings of Letterman's last week. On the 3rd show before he signed off, Eddie Vedder was the musical guest. During that segment, they departed from their usual shots and had some side shots of Will Lee. Right behind him was a good-sized Hartke amp sitting on top of a 4x10 Hartke cab with the aluminum speaker cones.
I thought that was worth a note here. His website says that live, he uses a Hartke HyDrive amp system. The website shows a combo but the Letterman gear is a separate amp and cab.