Hello, sorry for necroing this thread, but I figured I would post a solution for those still looking for one.
Introduction first: I'm an amateur with good experience in electronics. I am not, however, an audiophile or audio technician. Hence please take my results with a grain of salt.
I managed to repurpose the dual low-pass filters at U4 (previously used to filter out DAC electrical noise) into separate low + high pass filters, one for each amp. I designed them as second order filters in order to have a steeper roll-off at 40db/decade (or 12db/octave).
Assembly is quite fiddly and requires shuffling quite a few SMD components, not for the inexperienced, but it can be done!
I'd also say the speakers now sound as fine as they can, given they weren't great to begin with (but I got mine broken for 50€, so can't complain LOL!)
I tried documenting the repurposing process in detail, hopefully anyone with understanding of the matter should be able to replicate my results.

Moderator edit: New thread created from this.

 

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Correction: I erroneously soldered the input capacitor for the high pass filter to ground.
It must be soldered to what was the output of the DAC section of U3. You must first remove the electrolytic capacitor on these two pads and cut the trace from U3 to the square pad; now you can use the square pad to connect the signal and input capacitor for the high pass filter. The process for the low pass filter is the same, just with series resistor instead of capacitor.

 

I ended up adding a little clipping indicator circuit for the low-frequency amp based on this design:
https://www.hackster.io/simpletronic/clip-indicator-for-any-power-amp-10152d
it works pretty well and I soldered it to the O.L. LED on the original board!
I replaced the biassing diodes with a single LED as it has a forward voltage similar to that of the 3 diodes shown in the original schematic. I also had to add a signal diode to the input coming from the amp so that only the positive half wave can pass
I took power from the main PSU (soldered wires to +28V and GND, we don't care about -28V) and soldered a wire from the output of the LF amp to the signal input of this circuit.
Finally, I soldered the output of Q2 to the O.L. LED and cut the original traces to it to make sure I was not disturbing other parts of the board.