The 1070's DVI switching makes no changes to the signal. It's DVI switching is there as a convenience for cases where your display has fewer DVI or HDMI inputs than you have DVI or HDMI sources. The component switching is similar (convenience) with the secondary convenience benefit of transcoding composite and s-video sources to component.

Your description of what the Motorola box is doing makes me think that it isn't properly pillarboxing 4:3 content, something which needs to be done at the cable box. The Sony (since it has a cable card interface) would be handling that internally rather than relying on an outside source to do it, which is likely why you are seeing different behavior. I'd check your Motorola box's controls some more, as something may be there to allow this adjustment.