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.
Here is a raw frequency response graph for the 1990s (Oratory1990) with the analytical pads on:
To it I’ve added an approximation of the balanced pads as a purple line, based on Rtings' measurements. If a frequency response graph is totally meaningless to anyone, frequency or pitch from bass to treble is on the left/right axis and loudness is on the up/down axis. The broad green line represents one concept of what an accurate/flat/neutral frequency response should be. So in theory, wherever the orange or purple lines rise above the green line the DT 1990 is making that frequency range too loud. And wherever the orange or blue line falls below the green line the 1990s are making that frequency range too quiet. (But in fact the green line's bass elevation on the left and the upper mids rise on the right that peaks around 3 to 4 kHz are both highly subject to personal preference.)
I’ve also shown the four Loki control points as red arrows. We know what frequencies they centre upon. We know how much they can increase or decrease loudness — more than enough for this task. What we don’t know is how broadly each control point’s effect extends (which is known as the Q factor in EQ-speak). But it seems likely they have a wide effect rather than a narrow one, extending at least half way between one control point and the next.
The 20 Hz or sub-bass control point (left-most dial) is just fine. You can raise or lower it to taste and depending on whether you’re using the analytical or balanced pads. The 8 kHz or right-most dial is also ideally situated to tame the 8 kHz loudness spike that bothers a certain percent of the population. That said, it will probably influence a wider range of frequencies than would be ideal.
But the middle two dials are sub-optimally placed for adjusting the DT 1990. What we really want is a control at 200 Hz with a broad range of effect (Q factor) and another around 3 to 4 kHz with a moderate range of effect. But the provided 400 Hz and 2 kHz controls are too far from the required frequencies. So they are as likely to improve some frequencies while at the same time making other frequencies worse when used with the DT 1990. How problematic, if at all, that is for you will likely vary from track to track and genre to genre.
Going back to your first point you wrote:
> 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, 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.
(<< That's very obviously not a DT 1990, but he doesn't care; he's bopping along to the bass)
