Can you try to compare bass slam of the OG HE6 with just holding the drivers of HE6SE a few millimetres over your ear?
I had the OG before, and the first thing I noticed missing on the HE6SE was the slam, but after some experiments (sealed the ear pads, deeper pads…) I realised that the slam was hitting much harder whenever I removed the headphones. And just "hovering” the drivers over my ears gave me some resemblance of my memory on the OG slam.
I went with installing permanent fixed “bass port” to the plastic that hold the ear pads in place. Just used some empty wire insulation, 3 ports was my sweet spot, you can try with 1 or 2 first and see. When I applied some low shelf EQ, the slam was as good or better in my memory compared to the OG.
4 ports was to much with EQ and experienced some diaphragm “flapping” (hitting magnets because it moved to great distance)
Easy to check if you like it before going on with the mod, just “hover” the drivers over you ears.