The 80 ohm models sound just fine through the FM3 to me. I've tried the 250 and 80 ohm models and they are pretty similar with obviously the 80 ohm being louder. The 770's generally measure pretty flat outside of the boost in the high treble. I consider them neutral leaning clear and bright, maybe a little less midrange than you'd want for listening to music, and of course the boost in the high end.
I'm listening to music on my Macbook right now with them, volume around 20%. I switched over to my 250 ohm DT 990's and I had to increase the volume up to about 40%. So it roughly seems like the 250 ohm models are about half the volume of the 80 ohms. And to round it out, my 300 ohm Sennheiser HD6xx are about just a bit louder than the 250 ohm Beyers.
For open backs I think the HD6xx are a great value as they are like HD650's at 60% of the cost. Everyone seems to like or love them. If you end up really liking the Beyer signature sound, the DT 880 are a really good open back alternative that is in the same ballpark tonally although with a roll off in the bass. The 990 are similar to the 770's but a little brighter which is over the top. In contrast, the Sennheiser 6xx has a much bigger focus on the midrange all the way from the low mids up to the high mids, with a rolled off high end.