I have the 80 Ohm. I never had m50 but compared to cans like T1, HE-400i, SR-325e then the DT250 has recessed treble. DT250 has a mid bass hump then rolls off the sub bass. So I don't see where the extension is coming from. It's a unique characteristtic of Beyer cans but its bass extension do not even come close to T1. If you want to hear what a really good bass extension try any of the planar headphones.
I'd say DT250 has similar treble with T51P but with emphasized mid bass. I would put DT250 to neutral with a hint of warmness depending on what music is playing. It could be fairly neutral sounding if the music don't call much for bass response. But with music that has prominent bass, the treble immediately goes to the backseat.
The 80 ohm allegedly sounds a bit different than the 250 ohm; many people say the 250 ohm has better bass extension than the 80 ohm. It definitely has better bass extension than any of my open headphones.
Anyway, I got my replacement pair of DT250's and they have a noticeably different sound signature... The original pair had an obnoxious peak somewhere around 1 khz that made voices sound very annoying sometimes. The new pair doesn't seem to have that, and it also seems to have more bass and possibly slightly less emphasized treble. It also has a cord that has smaller diameter coils and the cord coating itself seems be less shiny than the original pair.
The original pair has a higher serial number which makes me think it is newer, but the replacement pair seems to sound better overall so I really don't know what is going on with these.