Room correction
Measure how the room distorts the speakers at the listening position, then compute a filter that undoes the damage.
A measurement microphone goes where the listener sits, a sweep is played and the software derives the room's impulse response. From that it computes an inverse filter, usually an FIR filter able to fix timing as well as frequency, and places it last in the signal chain. Dirac Live, Sonarworks, Audyssey in home-cinema receivers and Trinnov in expensive studios are all variations on the theme.
Physics sets the limits. A peak from a standing wave can be pulled down, but a null where waves cancel each other cannot be filled — boosting there only burns headroom and cooks the voice coil. The correction also holds for one spot; half a metre to the side it is a different room. Acoustic treatment first, filters second.