Yeah the no.1 mod here is chucking out those 0.0022uF caps on the outputs (to ground). When they've gone, the soundstage appears. I suggest do this first - if you don't notice any difference then you're really going to be wasting your time doing the rest because the other mods are a bit more subtle than this one.
Another mod I've done which I consider definitely worthwhile is restoring absolute phase. The TDA1543 inverts when used with passive I/V because the datasheet shows its intended operation is with an opamp I/V which inverts the signal. If you're feeling brave that's not a very difficult mod to do but it does involve lifting some pins on the SO14 device near the switch. Along with that I attenuated the I2S lines to the DACs in an attempt to reduce digital-analog crosstalk. Such mods have been practiced by practitioners of TDA1541 circuits over on diyaudio.
Next up is regrounding the outputs and inputs. This is more tricky because of all the ground fill copper cutting that's needed below the RCA phono sockets and power supply inlet, on both sides of the PCB. The aim is to reduce or eliminate common impedance coupling by enforcing star grounding. Having done this then digititus is reduced and the sound gets sweeter and more endearing to listen to. I regrounded the DAC 0V connections too, just to complete the effect. In general, PCB ground fills are a bad thing for sound coz you're never quite sure what currents are going where.
Lastly there are some mods which I do anyway which I'm not sure make a difference to the sound but give me a sense of satisfaction for doing them. This includes some passive filtering on the input to the DAC regulator, which means reducing the supply voltage (by about 1V) to the DACs.
If you need more explicit details on these ones, let me know and I'll endeavour to help. I might even take some new pics to show the latest mods which aren't on those pics already up.