The X2 will stick to the CEA-EDID resolutions it's told are supported by the HDMI device it's connected too. So the problem is probbly the amp which only has 1080i certified switches, and that is reflected in the EDID it outputs, even if the display supports 1080p. You need to check out a Denon forum, and see what needs doing to pass on the edid.
Devices which don't use EDID can output what they like, and you decide if what you see is acceptable.
P.S. The fact it's stuck on 720p makes me think this is a HDMI HDCP (Copy protection) problem. If the X2 does not get HDCP confirmation it will not output 1080i/p. Many office type projectors don't supprt HDCP, nor PC monitors. Many that do don't handshake it well enough. You might need to test with the X2 connected directly to the projector.
Hi, this is what I've done and apparently it works!
I switch on the med before anything else and then I start switching on denon and optoma; in this case I've the complete range of video resolution. I think is defenetly a HDCP issue but since I'm using a harmony one as remote I can change the order of the devices and fix the problem.