I see nothing special in the AD/DA circuitry, except maybe that they've built a successive approximation circuit with more "discrete" components, most likely as a cost saving measure because 12bit ADCs for audio rate were not exactly cheap back in the days, plus they needed one that supports the Z80's 8-bit bus.
To me, and also from my own experiments, this is where the real difference in the SP1200's sound comes from:



It's one of the weirdest and most hard core ways I've ever seen to re-pitch samples during playback, producing artifacts which, in combination with a relatively low frequency anti-aliasing filter, makes the machine sound dirty in a somewhat pleasing way.

 

The other from wavetracing sounds also quite nice:
https://wavetracing.com/products/dq5010

 

Dear Jebusina, that's evil genius! You can tell that's oldschool DSP geeks at work, not many people outside that discipline would even think to pitch samples by accumulating a counter then advancing the sample pointer on condition. I wonder if that note table for tuning is available somewhere, a bit like how the tuning tables in trackers have just evolved from the original Amiga code.

That just screams out to be implemented in code. This is a job for Reaktor Core or Max gen~ I think.