I agree, my experience too...this is a very noticeable and linear issue with these interface amps.
The volume dropping off that is, as the headphone ohms increase.
As for is a 250ohm headphone better sounding to me than my 120 ohm? I cant say, Id say NO, not really, because my 120ohm can sound fabulous too...both being open/semi open and at times I like them better than my 250's....closed backs are fine too, best for tracking with little 64 ohm Sen hd280.