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Not sure how the 4308 works but on older models you could only get the GUI on screen when conected by a composite or s-video cable.
I'm guessing you are only connected to the display by HDMI.
Does it state it any where in the manual?
I recently purchased one of these amps and in the process of learning how to use it I found that the GUI from the amp would no longer display (HMDI zone 1 to LCD TV). It worked flawlessly beforehand - I can only assume that whilst changing various settings I somehow managed to trunthe GUI off (is that even possible???).
In the end I did a full reset of the receiver and everything worked OK again - you may want to give this ago if you don't mind losing all your settings.
On mine i/p scaler set to off, I don't use it at all. GUI still appears over HDMI. Only disapears if I set sound to Pure Direct (shuts off all video circuits).
Taken from manual:
"When in the PURE DIRECT mode, the GUI screen is not displayed
and the display on the main unit is turned off.
• If the HDMI input connector is selected, video outputs are output in
the PURE DIRECT mode.
• The channel level and surround parameters in the PURE DIRECT
mode are the same as in the DIRECT mode.
Hope this helps.