Following up on my own question, here.
So, after reading and asking around, is it possible that the SR5006 doesn't upscale digital video signals?
So, if the input is HDMI, it's basically just pass-through signal? That would explain why my STB outputting HDMI 720p signal to the receiverout is output at 720p through the Marantz.
Then, what if I connected the STB in component to the Marantz instead? Would it be output as 1080p HDMI? Or still 720p HDMI? The manual really isn't clear about that. But the website and the reviews I saw everywhere all state "upscaling of ANALOG video inputs up to 1080p".