I would start with the overall equalizer assuming there is no fault with this 7k since the sound should certainly be better than the 5k, clearer if anything. Press Program Menus, Reverb & Effect, Equalizer. At FLAT the first slider should affect 125Hz from the initial state. Select that slider, move down to FC with the lower button, trying each range from 40 upwards to 300 or 400 Hz, moving back to the slider and taking it down 3 or 6dB etc to taste, you should be able to find the frequency range of the bass heaviness that annoys you. Naturally EQ ON has to be switched in the corner of the screen. If you find a setting that suits you can prevent new loads from altering it in Data Protection and/or different panel memories from changing it in the Expansion filter.

I'm not entirely clear what is meant with part-editing Customs but Custom styles generally need to be edited in the Composer. Press Program Menus, Composer, Custom Style Copy. Set the Direction at the bottom for Custom to Composer and dial in the relevant locations and OK. The style can now be comprehensively edited (including individual bass and part EQ etc) in the Composer, then sent back to Custom in the same way, with the opposite Direction setting.