Problem solved.

I changed 2 things, not sure what fixed it.

Firstly, I defined FAKE_BASSADJ and FAKE_TREBLEADJ and added them to the buttons table to allow ir_translation.

Secondly, I removed the copious amount of serial debug output that I had wedged into my eq write function. (I seem to remember that serial port is slower in DC mode?)

Anyway, I have working bass and treble control. Who wants it?
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Mk2a 60GB Blue. Serial 030102962 sig.mp3: File Format not Valid.