I have finally come to the conclusion that burning in wasn't able to cure my deep bass issue. The 600 ohm systems just aren't capable to produce as much deep bass as the previously installed 250Ω systems. EQing and padrolling helped a bit, but not sufficiently (and EQing is evil

).
However, I had another Beyer tinker session with my buddy NoXter yesterday, we compared his 600Ω
closed back modded DT880 with mine, and I ended up applying that mod (again) - closing of the housing except one
very small hole, paired with DT770 pads.

The tonality is unaltered and kept as snappy as stock, except the deep bass layer that gets the authority I missed stock. Only price you have to pay is loss of airyness due to the closed housing principle.
Finally we tinkered our idle old 250Ω systems back into work (obsolete DT880'2003 and Superlux HD660 housings), two outstanding rumbling Ambient & Post Rock machines, but of course dead slow and a bit cold in comparison.
Both worlds in coexistance.
