OK, now I'm comparing the 770 pro 32 ohm with the 990 pro 250 ohm, which just arrived.
The 990's are more detailed and brighter with details that may be overwhelming for listening if not EQed properly.
The 770 pro 32, are closer to what an average consumer is used to but clearly better than anything he/she tried, while the 990 250 ohm, sound like a step forward as details go. Every sound is tighter, detailed and precisely delivered, which for auditions may be to much ! So, if you want some really good headphones, easy to drive, go for the 770 32 ohm, they're really good but if you're a freak and don't care and wanna also buy a DAC and EQ them and so on, then go higher, for either the 80 ohm 770 or 250 ohm.
Listening to either of them 770/990/880 at 250 ohm, out of a phone is pure garbage so totally forget it. Whoever tells you they work is a complete noob.... yes they do make sounds, but you have no bass and you can only hear very good highs, which sucks. For mobile, if you wanna listen music properly then get the 32 ohm as they basically are 100% superior to the 250 ohm without an amp !!! Whoever tells you anything else is either a liar or a troll who never tried or something. Straight out of the phone or out of laptop, these 250 ohm sound non impressive. So, again, i say... if you wanna listen straight from phone, then get the 32 ohm. The 250 ohm NEED amp and 80 also. With an amp, yes the 250 ohm sound better indeed, with more details but the difference is not stellar as some may think. The 250 ohm, is clearly a better tool though for those who use them for making music.
Listening to both, 32 ohm and 250 ohm now, I'm really impressed by the 32 ohm 770's. So easy to drive with such good sound, this is rare indeed.
So, to sum it up, get higher ohms if you have or wanna get an amp, else go for the 32 ohm and you won't regret it.
Good luck!