They are very load dependant. As the impedance drops, the top frequencies are shelved. Most chip amps do not like a drop below 6 ohms. Perhaps you may need to look at the impedance curve of your loudspeakers AND cables and see where the dip is and see if this is recessing the top. those amplifiers with proper load provide exceptional phase and bandwidth power response.

Look at the impedance curve.

Wish you well fellow!