If you look at the distortion of most amps it rises sharply at higher frequencies.lamm set amp produces uniform distortion k2 across the entire audio band. That leads to this impression. If he were to create more or less distortion in the treble, the impression would disappear. It is probably intended to keep K2 high in order to create the sound character. Blue 16ohm/ 8ohm magenta/4 ohm red tap.View attachment 119102
If you need more control over the speaker, you should use the 4 ohm tap dampingfactor high.