Don't let the play of words fool you. Digital does not mean High Definition. Digital is just that, the conversion over from Analog broadcast systems. High Definition is a totally different animal, and the only way to receive those channels with the HD box is to get HD service, otherwise you will just hear the sound, but no picture.

I would just save on the box and just use your QAM tuner for the digital channels, unless you want the premium HD channels like HBO HD. That, and buy a blu ray player, so you really experience true, full HD withouth downconversion.