I had all three of them (600 Ohm) for testing purposes since I also had no real idea which one to get. I wrote down my opinion here
but it is all in German - maybe you want to use Goolge translate. I can give you a short summary of my personal opinion about them.DT 770
It isolates quite good from your environment but don't think it also works in the other direction. I was very surprised that a lot of sound escapes so don't think you can enjoy some music while your girlfriend is in the same room and wants to watch TV
For a closed headphone it has a nice but limited sound stage. Of course it can not be compared to real open headphones so I would say for a closed one it is good.
The overall sound was not really my thing. For me the mid-range was to over-presented, almost annoying and exhausting in some tracks. The 770 also sometimes tend to add a very very slightly echo/hollow to some songs - hardly audible but it is there when compared to the other ones.
The bass is a special thing... it will depend A LOT on how you wear these cans. If you push them a bit towards your ears the bass is good, but when you move them slightly away from your ears the bass loses a lot of impact. If you pull away the headpohne to the sides - but still touching your head with the velour pads - the bass is almost gone
I know that many people prefer the 770 Pro which is designed for usage in studios and I guess this variant is the better one because its headband is stronger and pushes the cans on the ears so the bass should be no more problem here.DT 880
If you search for reviews on this one you will always come across one thing which is absolutely true: they have a very neutral and linear sound. You can listen to them easily, no range is over-presented. Its neutral and analytic sound could also be a problem for some people because they might find it just boring.
Compared to the 770 and 990 the bass is clearly less pronounced. It could be taken as the serious and natural bass but I really like a good bass that has some impact so I did not really enjoy them because it always felt like something is missing. Don't take it wrong: the overall sound is great but seems a bit lifeless.
If you don't mind to play around with an equalizer the 880 could be interesting since it has a natural base and can be pushed in any direction to your needs.
The sound-stage is a bit wider. It is somewhere between the 770 and 990.DT 990
Well, I really have to say that I fell in love with them after the first beats
They have everything that the 770 and 880 are missing. The sound-stage is wide and you feel like in a bubble of sound in which you want to go deeper and deeper. The overall sound has some kind of atmosphere - it is hard to explain, the sound feels much more lively compared to the other ones.
One thing that is outstanding on the 990 is its bass. Damn, it is so unbelievable deep and has so much impact. Hard to imagine that this comes from an open headphone.
I'm pretty sure that its strong bass could be too much for some people so this one is only recommend if you like a strong and deep bass.
The overall sound has some bathtub characteristic (imagine a frequency curve): a lot of bass, mid-range is a bit recessed and the higher ranges are a bit more present. I really like it and could listen to music with it for hours.
People who like a neutral sound should stay away from the 990 since this one was made for fun.
The 990 also has a very constant sound, no matter how you wear it. If you pull it slightly from the ears away the sound gets only quieter but does not really change in its appearance.
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Keep in mind that this is only my personal opinion. If I would have to rate them:
990: 10/10 - just a great and bassy sound experience which I never want to miss again, feels like it was made just for me
880: 8/10 - good and very neutral sound but tends to be a bit boring and lifeless
770: 6/10 - if worn "correctly" they sound OK but the over-presented mid-range and iffy bass made me crazy