Why does GLM not fix dips on frequency response?
Often dips in the response are caused by reflections. Direct sound vs reflected sound from ceiling, floor etc. are out phase at certain frequency. If you boost this frequency, also magnitude of the reflected wave increases. Thus efficiency of boosting dips is not good.
Using parametric filters with positive gain to compensate for dips can have following negative side effects:
-Loss of headroom, as there is more gain at boost frequency. +3 dB means double power.
-Dip location and shape depends on listening location. When listening location changes, center frequency of the dip can change. At worst case, there is a peak at same frequency where the dip used to be. Which is amplified even more by the boosting parametric filter.
-Positive notch causes ringing in time domain. This is more likely be audible above low frequencies and high Q-values.
-Unsymmetrical notches will change phase response of the speaker, compromising stereo imaging. This is more likely to happen above low frequencies.