I'm a bit unsure what some of your questions are asking, but I'll try my best to answer. I have the AVR 791. It has great sound quality, and the Audyssey function is great for auto calibration.
Now, from what I understand, the only differences between the 591 and 791 is the number of channels, 5.1 vs 7.1 respectivley, and the usb input. Other than that, the features and sound quality is pretty much equal. Though I have never personally used the 591.
1. I personally never use the usb input. I just play movies and music through my other sources. If this is a needed feature for you though, then the 591 doesn't have it.
2. There is upscalling of all SD sources. It upscales and "upconverts" as good as any other device that claims to do so. Just remember it can only improve image quality slightly. It can't turn SD material into HD material, it can simply make what is there, seem fresher.
3. There is HDMI stand-by pass through. In order to have it pass through content, you must turn on the receiver first with the video source playing, then turn off the receiver. The video source will remain on the TV, while the receiver will be off.
4. I'm unsure what you're asking here. But if you were to buy a Blu-Ray player with all of those features, then your combined setup of that Blu-Ray player with the Denon AVR 591, would be equivalent to a higher priced receiver with those features. Though the higher priced receiver wouldn't have the Blu-Ray Player.
Overall, the 591 is a good receiver with great sound quality.