Interesting thought, thanks.
I personally find the HPF almost useless; and to make matters worse it even
affects the sound slightly when the slider is in the bottom position (you
can see this with an oscilloscope).
Anyway, the important thing to keep in mind for the HPF is that it can only
work on material that contains suitable frequencies. Compare it to the LPF:
If the source material is reasonably "buzzy" you will always get some
filtering result because the harmonics continue upwards anyway.
With the HPF it's different: If you play a somewhat middle to high note the
HPF will have nothing to pass for quite some time.
So please test this once again with very low notes (transposition two
octaves down) and check if the response now starts earlier.
I tried it with the lowest possible frequencies, and it's still the same.
I'm just wondering if this is common with the Mark IIs -- I notice that the circuit on the older machines is slightly different, and while I've repaired a number of Odysseys, I've never really paid any attention to the HPF.
Maybe the solution is to build in some kind of active HPF...
~GMM