i guess that the already exceptional performance of the kh150 in terms of low distortion could even improve with a low cut filter adequately set
small woofers are stressed by lowest frequencies
i would like to see the distortion plot at 106dB/1 meter with low cut equal to the xover point of the KH420 (i. e. 570Hz)
Harmonic distortion plots are sweeps, only 1 frequency is playing at a time so crossovers or filters would not change anything. You're thinking of IMD which is not tested on ASR.