Just to tie a bow on this thread, I got the Neumann sub and have no regrets. I've gotten used to it now, but I recall my initial subjective, non-scientific reaction was that there was greater stereo separation after adding it to the 310s--a wider and more complete image. (Could chalk that up to emotions and spending the $$$ at first, I know. I get it).
After almost a year with the sub, I can say that there weren't/aren't any problems with bass nulls in the room. I never thought the 310s on their own were particularly lacking in bass to a degree... just not low or crispy or punchy enough. After adding the sub, familiar program material sounds the "same," but I've caught a few of my own recordings where I didn't high pass some tracks and the sub made it really, really obvious that the tracks had problematic low freq content/rumble that was inaudible before.
In addition to that, the sub is most apparent when playing cinematic material (duh) - like Blade Runner 2049. The sub brings out low frequencies that were not pushed out before. Am I recording soundtracks? No. But I do listen to them and record synths and electronica. The sub has made me more comfortable playing back and catching any issues.
In sum, it ended up being a really balanced setup (prob by chance, but I'll take it). The sub kicks in well when there's some deep content, but has not been overwhelming at all. I suppose most people could do without it (high pass everything!!!!), but I feel much better being able to hear the low lows when they're there, intentionally or not. Hope this is helpful and thank you for the input!