Thanks everyone for the suggestions and help. I'm not 100% sure what the problem was, but I think everything is working now.

I cycled the power on the Motorola cable box as sluggo suggested and triple-checked all of my settings on the 1070 and the cable box. Thanks gonk for letting me know that 3/2.1 indicates a 5.1 signal. (I expected to see "5.1" on the display.)

I tested the system today with a HD movie on HBO as well as the Cubs game on WGN-HD. Both were broadcast with DD 5.1 audio track and both were correctly routed through all five speakers plus the sub.

I'll have to wait until Sunday to see if everything works for The Sopranos, but for now, things look -- and sound -- good.