There are effects which add frequency content, and effects which take/carve it away. Wahs, autowahs, EQ, phasers, and Uni-vibes (and the Worm) fit in that latter category. These can be placed before OR after an effect which adds frequency content. If placed after, they dictate what of the existing (and added) frequency content is allowed to pass through. If placed before, they can determine how much or what kind of frequency content is added. This also changes the tone, but in a different sort of way.
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