This is easy to answer: KH310D does not work digitally with the KH750, you have to connect them analog to the KH750. So no reason at all to chose the D if you want the KH750.
I think it is described in the manual of the KH750, i suggest reading it:
https://www.neumann.com/downloadcenter/download?assetId=12502
Only the KH150 can be connected digitally to the 750 at the moment (i guess the upcoming KH120II as well).
Thanks for the quick response! Actually I read the manual, hence I was asking, because page 11-12 diagrams says you can have all connection variants. However, good point, that I missed the KH 310 manual (I just read it again), where you can read from the diagrams that there is indeed a delay compensation benefit on its own, but not used in MA, since these speakers are not network enabled. (It may be useful in surround setup or to lip-sync tv if it’s far away… so digital connection is useful too.) Additionally, even if 310 is digitally connected to the sub, DAC happens in KH 310 D anyway before the usual back panel options, that you can find also on analog speaker version. Silly question from me.
In a nutshell, if there is any alignment happens in these speakers, KH 750 makes it, regardless the connection. The only benefit of full digital is when you connect the source to sub via digital, saving one single conversion. Which means I should just go for the cheaper/better conditioned second-hand 310 pair.