There is very little content I see which doesn't fill my screen unless it is older 4:3 content with bars on the left/right of the image, or movies in the 2.35 aspect ratio with black bars above and below the image. I don't see any content with black bars both above/below and left/right of the image.
If you are running into aspect ratio issues constantly, then there may be something else going on and it could be a resolution setting in one of the devices. HDTV content should fill the screen completely, like the menus in Apple TV or from your Blu-ray Disc player. If you constantly have black bars all around, then I would connect the device DIRECTLY to the TV, bypassing the AV receiver, and go into the resolution settings of the device and ensure it is setup properly. It takes some time to do this testing, but at the end of the day, you will see if the Denon is adding some issue along the way, or if the source is causing the problem.
If the complaint is that 4:3 content is 4:3 and there are black bars. Or 2.35 content is 2.35 and has black bars, then this is something that you guys just need to deal with. That's the way the content was shot, so it is how it is being presented.