The DAC in the PCBSNR (and many other similar devices) is a simple passive lowpass RC-filter. The frequency is set by the output resistor R42 (1k) and capacitor C15 (47nF). Together they give a -3dB point of 3386Hz. This is somewhat conservative, but the slope is gentle. If you want more treble, swap the capacitor for a 22nF from the tilt (giving -3dB @ 7234Hz), or a spare cap of e.g. 10nF. If you want less DAC noise and cleaner sine waves, swap out one of the 100nF capacitors for the 47nF one (resulting in -3dB @ 1591Hz). Just put it in one of the 100nF spots.
The two tilt filters in System Sound share the same tilt topology, they only differ in values – one has 22nF capacitors, the other 47nF. By swapping capacitors, you can easily get different equalisers.
Links to the two channels: mic + in1 and in2 + osc. As you can see (if you move the slider) they tilt around different frequencies (around 400 and 800). By swapping one or both for a 100nF capacitor (just use the one that was in the tilt where the 100nF capacitor was supposed to go, it will be fine) you can get other shapes. For instance an EQ with a scoop, a normal tilt with a tilt frequency, around 180Hz, or this thing?? (I can’t recommend that last one, though I haven’t heard it).
Sweep of the left (mic + in1) tilt filter