I have the KH 310 and bought a KH 750 DSP last week. The subwoofer acts as a DSP for the whole system. If you use the iPad-app, you can specify which Neumann-speakers are connected. It supports KH80, 120, 310 and 420. I can set 8 fully parametric EQs (or other filter like hs, ls) for EACH of the three speaker (L, R, Sub) individually, if i want. I also can use the iPad as a remote volume control of the whole system. I can switch sub on and off via iPad and so on. The KH 750 even adds FIR linear phase filtering to the analogue mains. As far as i know it works the same if you combine KH80 or 120 to the 750.
I´m not sure if KH120 will have any advantages over KH80 if combined with the KH750. If you buy the KH80 you have all the DSP/App-features without the sub, if you don´t want it.
Info and screenshots from Neumann here:
https://en-de.neumann.com/kh-750-dsp
Here is a review and measurements of the KH120:
https://en-de.neumann.com/product_files/1711/download