sorry to break the bad news....Letterman tonight is 5.1, all channels reporting for duty on ch. 290, CBS Seattle. The surrounds are a little weak, but center channel is loud and clear.
maybe there's something wrong with your channel map - it's the only thing that makes sense.
PS...I listen in straight 5.1 - no surround effects or stadium nonsense, so I know it's good. Yamaha RX-V871 receiver