The Loki question seems to have two parts. A) does it degrade the 1990’s sound and B) will it make the appropriate adjustments to its tuning? I can’t answer A, but B seems pretty straightforward.
> The EQ points look ideal, I would try +3dB @ 20Hz and -6dB @ 8kHz.
As long as you are only looking for a two-band adjustment, the Loki's control set should be ideal,
I love my DT 1990, I don't want to change the sound profile, but I would like to tweak it. Some poorly mastered tunes do not go well with the Beyer top end. The DT 1990 with Analytical pads is just that, analytical and revealing, a great tool for mastering, but can be unforgiving.
Deeper bass is fun if not overdone, and as long as the drivers are capable, which I expect the Tesla drivers are. I am not a believer in the committee-developed, bass-tilted Harman sound profile. It is, as you've said, subject to personal preference.
Loki's control set should be ideal, except that the 8 kHz dial will likely have too broad an effect. The middle two dials may tempt you to play with them, but chances are you'll end up leaving them alone.
"Technically, Loki is a single-gain-stage active EQ with non-constant-Q passive filtering. In more English-y terms, that means that each band varies from broad to narrower as its gain is increased—small turns of the knobs result in broad, shallow changes, and bigger turns result in more narrow-band changes." https://www.schiit.com/products/loki
That seems an interesting way to tackle the Q issue. I would not make heavy adjustments. Anything beyond 6dB is going to cause phasing issues. I'm pretty sure I could tweak the FR to my taste, the significant question being, will I lose transparency? Most seem to say no, so at the very least any compromise would appear to be minor, and hopefully a worthwhile trade, as opined by others here.