UnitedPlugins is discounting SubBass Doctor 808 - the plugin designed to treat the sub frequencies unheard on some systems, to €29/$32 (50% off) until the 5th of June, 2022 (Reg. €59/$63).
Treating the lowest frequencies is not an easy task. And in some listening environments, it is almost impossible. If you mix while travelling or your mixing room or your system is not 100% you probably need SubBass Doctor 808.
Check the free 15-days fully working trial version of SubBass Doctor 808 to if it suits your workflow.