Of course you can listen to the DT 1990 PRO mkII and DT 1770 PRO mkII without EQ/filter but it will have some sharpness and 'hyper detail' if that's your thing.
The main difference between them is mostly in the bass and lower mids.
Only buy DT 1770 PRO mkII when you need a closed headphone (isolation) or want more bass (without having to resort to EQ) otherwise there is no need to own both.
Soundstage = same, treble = same, clarity = same, bass differs.
I'm sure there are people going to enjoy the stock sound and others may not like it (sharpness).
I modified my DT 1770 PRO mkII (requires drilling holes, adding some toilet paper and a filter) but with EQ you can do almost the same.
Lowered the clamping force and use a different (lower in microphonics) cable making it a far more enjoyable experience for me.
Result below... very similar to the (filtered) DT 1990 PRO mkII.
You can 'change' the stereo imaging/soundstage
a little by using the coiled cable instead of the straight cable.