You say you don't like the sound of the 4500 in a different and non-treated room with different speakers. Since it's usually the room and speakers that make the biggest differences compared to electronics, have you tried the 4500 in your home theatre in place of the 6005? It's quite possible the difference in sound you hear is due to room/speakers.
The Denon 6500 is just a higher model of the 4500 so I would expect them to "sound" pretty much the same.
You also don't say whether you are using Audyssey or not in either room.
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