Multi-band systems split the audio frequency spectrum into several overlapping frequency bands. Gain and ratio, usually even attack and release times can be set separately for all of these frequency bands, resulting in independent processing for each band. Recombining the outputs of the multi-band stages can lead to severe problems. The spectral balance of the summed output signal is almost always deviating from its input.
Thus the resulting relation of high, mid, and low frequencies may lead to a completely new impression of spectrum and sound quality. This is especially a problem when listening to music.