Turning EQ off just means your speakers are not using the room correction EQ curve calculated when you ran the setup. The Audyssey reference curve I think Denon just calls it “Audyssey” tries to correct for room speaker interaction to give you a flat frequency response but near the top end it roll off I think John mentioned at -2dB/octave. Not a huge amount but noticeable to me. The “Flat” curve tries to maintain a flat frequency response all the way across.
Some people like the high frequency roll off because some recording have a high frequency boost or are just poorly mastered and this sort of tames them. I don’t like it because much of the music I listen to has a lot of ambience which the roll off ruins for me.
