There are a few things to know about Headphones. The Ohm Thing. The Sensitivity Thing, and the Max Input level Thing.
The last one is how loud the cans can get, without distortion.
the sensitivity is how effektiv the input signal will be transfered by the headphones. for example. a Headphone with say: 32Ohm and 112dB/SPL is about double as loud, as a Headphone with 32Ohm but a sensitivity of 96dB/SPl. So the Sensitivity is important.
On the other Hand; if you have two Headphones, both with the same sensitivity of say 96dB/SPl but headphone 1. with an impedance of 32Ohm, the 2. with 80 Ohm, the first one will be much louder.
The Headphone amp of your UR28M is to weak to drive headphones with 250 or 600Ohm loud enough. Therefor you´ll need an external powerful headphone-Amp or so. Maybe a Headphone with 80 Ohm might be O.k. for your needs but…. this is a very personal preference and you hafe to try for yourself.
-> You can use headphones with less than 40ohm, lets say 16 or 32 Ohm with your interface without any problems.
The Spects for your interface tell you, how loud they get with an headphone with in impedance of 40ohms. But how loud are 35mW, and on witch frequency and and and. so check for yourself or get informations about witch headphones are used by other people with an interface like yours.
For example, for tracking i use cheap AKG Cans, with closed backs, 32Ohm with 112dB/SPL a they can get loud enough for this purpose with my UR22 MK2.
For Mixing, i use monitor speakers, and late at night my 32Ohm 92dB/SPL open Cans. For Mixing they are loud enough for me! For Tracking they have not enough power for working with microphones, and they are open so they would bleed to much sound when working with a microphone. For playing line in signals, bass guitar, or Keyboard they are loud enough for me.
So, as higher the impedance goes, as more problems with volume you can run in to. and don´t forget about the sensivity.
Long story sorry. and as often with audio stuff, not that simple. but i hope you´ll understand what i was trying to explain here, and yes! english is not my nativ language!