I would also say the 250 pro is better than a 400. When I have compared the 250 and the 440, I liked the 250 more. It has a more refined sound. But that may depend on the speakers.
It might depend on the power, yes.
On the other hand I have the feeling, that after my upgrade to the 1000 some kind separation in the stage is available again, I actually missed with the 250 Pro and I liked so much with the 400.
And yes: the sound signature of the 250 Pro is different (I like both though). I even have been happy with the 120 some years ago.
I would expect that a 440pro sounds better (al least equal) than a 250pro: more power and the benefits of a mono amp.
Here my choice would be the 440pro. The 400 is a different story, the improvements of the pro platform are much bigger than the differences in power. At least for 99% of all speakers...
With a 1000pro you are on the safe side :>)