Let's start with an example of a common Frequency Clash. The bottom end of an Acoustic Guitar vs a Bass Guitar. In this case it's wise to Hi Pass or Roll off Bottom end on the Acoustic Guitar since that's where the Bass mainly lives. Having additional bottom end from the Acoustic Guitar is just going to muddy up the mix and add unnecessary level. Usually with an Acoustic Guitar in a Pop mix The acoustic takes on the role of a rhythmic element like a shaker or hi hat so you don't want all the bottom end coming from the sound hole of the acoustic! Better to tailor it to work with the Bass.
