With two subs it's not any easier; you will need to place them carefully so as not to run into phase issues.
Then you will have a difficult time integrating them. And may end up dissatisfied even after putting a lot of effort and time.
The added cost is insignificant, and there is also a bundle that costs less than the sub and the MA 1 kit if bought separately.
I see, but the UMIK-1 is not such a big investment, and the WiiM Pro's main purpose is not its PEQ, so you won't be losing too much. OTOH, room EQ built into the speakers, and phase-coherent one, is much more robust and versatile than using an external solution.
Why do you need three outputs? You only need two.
Yes, the price difference is significant, I agree. In the end, it's your decision and your money. But I strongly suggest saving up for the Neumann solution. The KH 120 II are fine on their own, especially in a space. The sub will bring your listening experience to the very top, if executed correctly
My case is a bit different: the KH 120 do not go as low as the KH 120 II, and they lack phase correction and room EQ, so the KH 750 DSP brings all these benefits to my system, and the difference is indeed very substantial. In case of KH 120 II, though, not so much.