Nick, you can be sure that the "something" that sounds warm isn't the receiver when controls are set flat and the other variables, especially precise sound level, are controlled for. Properly controlled blind listening illustrates that competently designed power units don't add an audible sonic coloring. They simply don't have the substantial deviation from flat frequency response that would have to exist for them to cause sound to be "warm", "bright" or whatever.

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Enjoy the music, not the equipment.