DT990 emphasizes the bass and treble too much that it makes the whole midrange (where most instruments, foley, voices, etc are) feel distant and hollow sounding. It's not a balanced sound. That was my experience with them.

The DT880 is more balanced but also has a similar flavour of sound since it uses the same driver. I still have mine but I've slightly dulled the treble with a disc of felt to bring out the high mids more, they're pretty great now.

If you want a balanced mid/treble I would maybe look into getting something like that, or the Sennheiser HD560S or AKG K612, but if you want a bit more bass like the DT990 then I recall the Shure SRH440A and AKG K245 having that.

Personally I'd say the HD560s is a bit less neutral sounding than the HD600 because of its elevated treble and slightly distant sounding mids, although the bass and mid-bass balance is great and better than the HD600. I found that the presence boost and natural soundstage of the HD560s gives some instruments a boosted spatial sound to them, which you may or may not enjoy.

The HD600 in comparison is more forward and detailed sounding in the mids, with balanced treble that isn't hyped. It does lack sub bass and has a very slight mid-bass boost. All of which makes it sound a bit more closed in and analytical as a result. I use mine every day but the reason I like them is because they're a reliable boring workhorse as opposed to a fun or exciting sounding headphone.

Another headphone I'd recommend that is somewhere between a HD560s and HD600 is the AKG K612, which I think is AKG's best headphone by far.

It's just volume. Mismatched extreme cases of impedance can slightly alter the sound, usually for the worse by adding slightly extra bass. My powered Roland workstation's headphone jack is an example, it gets incredibly loud easily and makes some headphones bassier. Not desirable and an extreme case out of dozens.

Out of all the audio interfaces, amps and even dongles I've used in shops, studios and from home, they basically sound the same, just can deliver different volume.

Soundstage widening, extra detail, cleaning up frequencies, all of that I don't get and even if that was true, it's going to be a result of it altering the frequency response of headphones which is a bad thing. I don't want a coloured amp, I want an accurate amp.

Save your money in this marketing speak dominant hobby.