if you ask the question the other way around: what is the best headphones I can get for $400 if I do not want Mobius versatility, it's easier to answer - maybe ...
There are passive headphones with none of the extra features that Mobius has for $400 that do sound better -a bit - in some way.
Open back planars come to my mind, the Hifiman Massdrop HE4xx, if you remove the outside grill and replace it with something more acoustically transparent or leave the grill away (so you end up in the mod section).
But, they do not offer the step up of the sound if you use and like the 3D effect that Mobius has.
And there's less bass because they are open back and not active, as the constructor had no way to boost the bass. So you need an App to do this...
This is where you end up whatever other headphones you look at, Mobius is so special that it cannot be easily compared to other headphones.
So if you want to spend a bit more, let's say $1500, we can start to talk again - but wait, there is this 3D effect with headtracking …
Is there a car that can drive faster then my tuned 400 PS minivan?
Yes, but you cannot take your family to holiday's in it, they don't fit into the F1 racer, not to mention the luggage.
And you may not drive the F1 on public streets, and you have to change tires every 200 miles ...
... did I mention the F1 has no 3D effect?
Seriously, there are $ 400 headphones that have a slight edge on sound quality potential, but you have to build your own package from them - out of the box, not really.
I am right now in holidays and guess what I have taken with me from my huge collection: the Mobius and the Focal Clear.