The DT990's are an slight upgrade compared to the DT770 in both the bass and treble department. My only experience with the DT990 is the 600ohm version paired with a few of my modded class a hybrid tube amps that run on 12AU7 tubes sounds slightly warmer but has the slight shrill sounding highs that can be seen as bright not eliminated but more like covered up still sounding very good though.
The DT880's are not similar to neither the 770 or 990 as they have emphasis on the bass majority of the time.
The DT880 is more analytical and neutral pointing. They have the best balance of treble, mids, highs and bass out if the bunch.
As posted in another thread, the DT880's are similar sounding to the flagship Beyer headphones.
Hope my post helped.
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