I believe you are experiencing the placebo effect first hand.
No human is immune to bias.
The Onkyo also measures flat (20Hz-20kHz) especially in Pure Direct mode.
A “warm” sound probably means that the treble tapers down around 8-16 kHz, not flat from 20Hz-20kHz.
A “bright” or “harsh” sound probably means the the opposite- goes up or spikes in the treble region.
Thus, in Direct modes, the Onkyo and Monoprice amp are both flat or neutral from 20Hz-20kHz. Neither one is harsh or warm sounding.