I left the Thekk for overnight run and checked again. I won't go into the bass immediately but will do a more general _subjective_ analysis.
Firstly, Rögnir and Thekk are _very_different sounding HPs. But this is not a bass issue, it is the mids. They are simply forward on Thekk. Also the -6 dB drop in the 1 to 2.5 kHz region (which pretty much stays with Thekk). Or reading it differently, the pushed forward 50 Hz - 1 kHz region. For my taste (I like slight U or slight V tonalities with slight recessed mids) this gives a strange tonality, at least to my ears. This is more a vocal listening HP.
As you see below, Rögnir has a very different tuning. Especially notice the lower mids around 200 Hz (which gives mids clarity, but at the same time loses some mids body and the bass sounds slightly disconnected, which is the only _slight_ quirk in Rögnir's tonality), and the huge gap around the 1.5 kHz region. They are pretty much reversed between the two by about 6 dB, which makes a
huge difference in tonality.
Instead of patching different parts of the frequency band, this is how I fixed it looks like this:
which is
a bell at 280 Hz, Q (bandwidth) 0.63, gain -4 dB, slope 24 dB/oct (you may not have this parameter with every PEQ). Because we don't touch the treble, it might be a little too much for treble sensitive people, because you need to increase the volume to achieve similar mids level. But this will give you a much better bass and mids balance, with some nice mid-bass bump. If you want a Rögnir similar tonaliy, add another bell around 200 Hz , but I wouldn't recommend a too low gain there, but something like this:
which is
a bell at 200 Hz, Q (bandwidth) 1.9, gain -3 dB, slope 24 dB/oct.
Coming back to yesterday, why did I hear such a huge bass? Three things: 1) I possibly focused on upper mids / lower treble while adjusting the volume, which is kind of recessed. 2) TheHP has the capacity to go down quite well in the subbass region, and it does. It has a very clean and fast bass, and combined with forward upper bass / lower mids, I was kind of overwhelmed. 3) The break in process might (!) have helped with the tonality of a barely used 8 monts old HP.
I will keep both Thekk and Rögnir running for a while more.