Written together from various listening sessions - this is my personal feeling and opinion and not an absolute judgment!
Composer - Frequency response mids and bass almost straight. The bass goes far down, powerful, very dry sounding. Clear mids, fine definition. Treble without sibilants, almost silky like electrostatics, mids and highs finely marked. Stage wide, but slightly less spacious than K812. More demanding of attention. Wearing comfort, very good. Overall number one in this comparison.
DT1990 with analytic pads - bass doesn't go down as far, from around 100 Hz up to the treble the sound balance is about the same as Composer, not quite the fine detail and selectivity as Composer. Treble is sometimes overly emphasized (Beyer peak), but is only noticeable with certain pieces of music.
DT1990 with balanced pads - bass doesn't go down as far but has an increase in the mid-bass, which makes the sound seem a little warmer. The middle is about the same as with analytic pads, the highs are sometimes a little emphasized (Beyer peak), but this is only noticeable with certain pieces of music, not as strong as with analytic pads.
Overall the stage is a bit more intimate. Wearing comfort good to very good. DT1990 with balanced pads sounds warmer than the other two and is also suitable for long listening with suitable music or with a little EQ.
K812 with additional mesh - is the most open, wide stage sounding. Bass goes down very low but not as deep and powerful as Composer. Clear in the middle. Trebles also sometimes tend to be sibilants. The peak is about 1 kHz lower than the DT1990. Stage furthest. Comfortable to wear because it is heavy and the edges of the pads press on the skin.