performance of tri-amp vs stereo amp + mono amp

[American Pie]

Maybe you are looking for an EQ, so buy RME ADI-2, and you can change the sound to your taste.
You said you are using it as dual Channel now, so if one channel is not connected, the power supply delivers just the idle power to it, so about 10W, this could not be a limiting factor on a 1200W power supply.
And in 3 channels, 1200W of power in your ears would be a lot, so most probably enough to

 

[American Pie]

Don't understand why I would need an EQ. The sound is perfect through the Denon - in direct mode so without any EQ. Its that class D amp causing the harsh treble. But I'll try again with another brand later... Maybe a NAD M32.