Couple thoughts...
Jaakko's AutoEQ settings are based on a +4 dB bass boost, rather then the +6 recommend in the Harman curve. So his settings will reduce the bass a bit, which some may like, and some may not. (That is mostly to accommodate the open-back headphone users.) Imo, both Jaakko's and Oratory's settings are also a bit too forward in the ~1.5 to 2k range to be comfortable to my ears. And probably also a bit more rolled off in the very high frequencies than is necessary (which is also based on Harman).
I have the 250-ohm DT-770's, and use them on a daily basis. And one fairly easy solution to fix their brightness is to apply a simple slope function, using a Variable Graphic EQ in Equalizer APO's Configuration Editor. This won't really fix all the unevenness in the higher frequencies, but at least it will make the overall tonal balance of the headphones somewhat better, and more pleasing. (Though it will also dial up the sub-bass possibly a bit too much.)
An Example:
GraphicEQ: 20 0; 20000 -5
This creates a variable graphic EQ with just two points at 20 Hz and 20000 Hz, that should look something like this...
View attachment 158260
You can actually set the two points in the slope to whatever frequencies you want. But I recommend keeping them close to the above 20 and 20000 Hz values (which are based on the normal human hearing range) for convenience. And you should go no higher in frequency for the 2nd point than the Nyquist frequency, which is 1/2 your audio device's sample rate.
If your audio device's sample rate is 44.1 kHz, for example, that means you can potentially set the frequency of the 2nd point as high as 22050 Hz, but no higher. If the sample rate is 48 kHz (which is what I use, since I listen to alot of YouTube music videos), then you could potentially set the 2nd point as high as 24000 Hz. 20000 Hz is high enough for me though, because my high frequency hearing taps out in the mid-teens, kHz-wise.
If you currently have the bit depth on your audio device set to 16-bit, you should probably also increase that to 24-bit for better resampling of the volume changes with the EQ.
Use the volume control on your amp to adjust the overall volume level on the headphones. And adjust the steepness of the slope in the above Graphic EQ, by selecting the 2nd point at the higher frequency with a drag-box (right-click somewhere near the 2nd point, and drag a box around the point to select it, while holding down the right mouse button). And then moving that point up and down in 1 dB steps with the up/down arrow keys on your keyboard, until it sounds right to you... Very easy.
No Preamp setting is required for this, btw, since you should only be reducing the volume at the higher frequencies for the DT-770. And not actually increasing it anywhere with this approach.