Ahh! The plot thickens - or rather is about getting unraveled
Just tried an old 80's headphone, Denon AH-D300, a set of IP2 plugs, a set of Sony
unknown model EDIT: MDR-2808 plugs (printed on the plug) I found on the street.
Initially, all sounded crappy - until I realized that, when going through my Mackie monitors, I've been using the -20 db pad on my Luxkit Avance preamp.
I of course did this because else the output from my rig would make the Avance inputs overload, resulting in distortion.
Now, this pad is always inactive when using the headphone outs, just as tone controls are, so obviously, I get distortion.
The fun thing is that the Avance does almost useful distortion when the inputs are overloaded, which made me miss that point.
To remedy this, I turned the MP-2 Output down A LOT, and also turned the Room EQ to mid point, i.e. with no additional lifting of the 'body' and presence.
And waddayaknow.. things started to sound much better
The Sony plugs, that I'd expected to be crap, actually sounds the best.
Another matter of importance is that the headphone driver in my Avance preamp simply aren't geared the best for driving the more demanding DT770 cans, even if those are the less demanding 80 Ohm version.
Now, why on earth then do the Mackies sound so neat?
Well, the Mackies, being professional monitors, require fairly correct positioning, room treatment and listening distance, which is impractical in my room.
As such, I sit a Bit too close to them, and partly have a thin pad of acoustic filtering of the same type as is present in headphones in front of the very detailed titanium tweeters, and partly have those dimed 2 dB down.
As a result, when standing up playing a couple meters away, they sound just great with my slightly aggressive rig presets.
To finalize my findings, which could be important for others too:
My super-duper very linear and very revealing studio monitors have been setup not fully correctly.
And my, elsewise (new English word) fairly ok, Avance preamp weren't correctly matched to my rig.
To this should be added that a good quality real headphone amplifier should always be employed.
So, my presets were never fully correctly aligned to neither the preamp nor the studio monitors or the headphones.
And this has been further problemized (next new word) by not having real amps/speakers available for being able to check with reality.
I can't fully resolve these issues at the moment (due to the apartment frenzy), but will be able to do so in another life later.