Dear Karel: Other than IC and tonearm damping fluid level I think that could be worth to re-install ( like a new cartridge ) your cartridge just from the begin: clean the cartridge connectors pins, be sure that the tonearm to cartridge connectors make good connection, overhang, Azymuth, VTF, VTA, stylus deep clean, check that the cartridge cantilever is right on " target ", etc, etc.
It seems to me that the load impedance is not the problem. You could try with a different cartridge and see what happen.
If nothing works and I mean that you don't recovery that " bass " then you have to thing that maybe there is a trouble on the cartridge it self or on the phono stage.
Regards and enjoy the music.