I did some measuring with a measuring microphone and tried to get the EQ as linear as it gets easily.
There is a lot more work required, but I'm posting what I have since it will (probably
) be better than random.

Red is reference audio file before stereo system.
Green is uncorrected/linear.
EQ used for "4CH0-23" (not plain treble or bass used):
Now, this isn't all there is. Just going linear is not always the most enjoyable music experience. Bending the EQ strongly always introduces oddities that bring the speakers off their "natural" responses, and the EQ might or might not have phase issues. So in general you want to take this as a starting point and tweak it. It will be better than starting from randomly poking around.
In particular here is what I will do from here:
Hope this helps.
There is a lot more work required, but I'm posting what I have since it will (probably

Red is reference audio file before stereo system.
Green is uncorrected/linear.
EQ used for "4CH0-23" (not plain treble or bass used):
Code:
80 -10 # doesn't get the job done, maybe give up down there
200 +8
500 0
1k -4 # might be a mistake
2k +8
5k +8
10k +8
Now, this isn't all there is. Just going linear is not always the most enjoyable music experience. Bending the EQ strongly always introduces oddities that bring the speakers off their "natural" responses, and the EQ might or might not have phase issues. So in general you want to take this as a starting point and tweak it. It will be better than starting from randomly poking around.
In particular here is what I will do from here:
- As mentioned, the 80 Hz thing is drastic but doesn't get the job done. It is safe to bring it back up according to taste since it doesn't interfere much with the rest
- The 1k drop is the most drastic curve adjustment. Bringing it back up softly is a good thing to try
- The rest if EQ matching. Try adjusting every setting to 1/3rd or 1/2 of what it is right now from 0
Hope this helps.

