I don't mean to second-guess you, but are you sure that the bars are not in the source material? When I am watching HDTV, some of the material (and almost all of the commercials) are in 4:3 SDTV, which displays the black bars automatically.
When you say, "It displays non-HD material in 4:3", it makes me think that 4:3 is probably the correct aspect ratio for the material.
I apologize if I'm missing something obvious.
Technical Director, Home Theater Products