I bet you saw all those warm TUBES and figured they made the sound warmer. All in your head. Placebo effect.
According to McIntosh, the FR on that amp is 20Hz-20kHz +0.0dB/ -0.5dB, which means they might very slightly (0.5dB is VERY SLIGHT) taper the treble from 8kHz-20kHz. But I don't see how -0.5dB is going to make a significant difference.