With that experience behind me, I figured I knock out the resonance around 900 Hz which happened to also coincide with peak distortion. Wow, that cleaned up the distortion very nicely without hardly any impact on tonality. You could so easily hear it in vocals and even higher frequency notes. Transient notes because much cleaner and less grungy for lack of a better word. I need to record this effect some day so you all know what I am hearing.
Are you sure you were hearing a difference in the distortion of only 0.2% (1% vs 0.8%) or maybe you were hearing a difference because you reduced that peak at 900Hz?
Regarding your filter at 4438Hz, as you can see from the filter simulated response below, it actually dug quite a dip in that region.
To correct the uneven response in the 3kH-8kHz region you need 3 filters and then you end up with this, which looks nice and flat: