Dear Rada: Other what you already do it ( and you can go on to some limit. ) there are somethings that can/could affect the bass performance ( other than VTA that you already try it ): azimuth, VTF, on target cantilever align ( not the cartridge body ), precise tonearm set-up and overhang.
Of course that a different phono stage can/could makes a difference over your CAT but like you say this is: out of question.
You can try on the load impedance but if you go from 100 Ohms to 300-500 Ohms and you hear a difference in the bass performance other than a little change on SPL then you have to change that CAT because that kind of change on impedance IMHO can't/could't affect the quality bass response.
Regards and enjoy the music.
Raul.
Of course that a different phono stage can/could makes a difference over your CAT but like you say this is: out of question.
You can try on the load impedance but if you go from 100 Ohms to 300-500 Ohms and you hear a difference in the bass performance other than a little change on SPL then you have to change that CAT because that kind of change on impedance IMHO can't/could't affect the quality bass response.
Regards and enjoy the music.
Raul.