I did a lot of debugging on the circuit itself:
- touching up on all solder joints in the HH circuit
- checking every single path in the HH circuit (measuring conductivity)
- checking every solder joint through a magnifying lense
- measuring every resistor value
- checking that no electrolytic capacitor legs had broken off
Through all this I found a few suspicious solder joints, as well as created (and later discovered) a short circut. I also found three errors in the 9090 Hihat schematics, two of which does not matter and one that certainly do - however, the PCB is the correct one...
Anyway - I am not sure of any of this actually had much to do with the error. After taking a very close look at the various connectors involved in the HH, I discovered that a crimp in the HH tune connector was bent out of shape. It makes sense if this was the error, as it would affect both the closed and open hat. After replacing the crimp, I put the PCB back in the box and hooked up all cables - and whaddaya know, it works! :) Now, this was the last known HARDWARE error (excluding the rotation encoders that are just anoying, not really an error).
I also tried moving the headphone amp away from the circuit board to remove the noise (after all the previous supply line tricks failed to do so), however, this did NOT work. The noise in the stereo mix main out is not that bad though.
Now all I have to do is put everything back together again, and pray for everything to work once that is done :)