You'll never be able to fix the upper midrange hardness on the radio channels. It's like a built-in peaky distortion that is impossible to tame with the limited Skoda EQ controls. Setting the audio focus to "front" is about the best compromise, and have the subwoofer on maximum volume, with all balance and front/rear controls at centre. The only fix for me was to prize open the dashboard centre speaker grill and disconnect this completely, and then play all my music through a free music app with a built-in equaliser to pull out the offending 3.5kHz frequency. For iOS I use Onkyo HF Player and for Android the Jet music player, both free.