Use the source Luke.

The current bass/treble F/Q parameters are configurable. But it's difficult to do so since I never completely reverse engineered the DSP coefficients, just enough to make it work. But if you set EQ_DEBUG = 1 in empeg_mixer.c then when you adjust bands 9/10 in the players EQ (with bass/treble disabled) then you will see the coefficients on the serial port. IIRC, the first word will be the frequency and can be plugged straight into config.ini (Don't forget to prefix with '0x'). The second word is Q and dB combined. Make sure that dB = 0 when you take this reading. Then you want to take the high 16 bits of that word, postfix 00 to it, and plug it in to config.ini. eg if the word is 0x1234, you want to use 0x1200. config.ini labels are in hijack.c.

But hopefully this is all going away soon, to be replaced by the all singing and dancing DSP-based implementation. This should allow the f cutoff points to be adjustable, possibly also allowing the choice of 1st/2nd order filters. Who knows? First I have to iron out some bugs in my sinewave code.