I can't say anything about the Beyer 770, except that it has a good reputation of having a good bass. Pro 900 is a good basshead can (edit
, although I have to add that as a basshead, it would probably become somewhat of a letdown. I'm not sure if it's my equipment before the headphones, but I can't crank the bass up too much because of the resonance. Not EQ:ed it takes the cake for bass, but EQ:ing it is counter-productive IMO. In that sense, my HD25-1 II is a true masterpiece, you can EQ that can into a
leaf blower and it would still ask you for more.
Where can you find a Pro 900 for 250 euro? That is a very good price for the Pro 900.