RES (Resonance): also known as "Q" of the default filter. Resonance increases the chosen part of the filter cutoff. Kind of a "hump" in the spectrum. You hear this as "sharpness" versus "dull" expression of the sound. Too much resonance causes distortion, and can harm both hearing! and speakers. The default presets in the ZynAddSubFX folders should only use moderate Q, because the internal ZynAddSubFX Q is quite high! Serious clipping from factory ZynAddSubFX presets will often be solvable, if you moderate the Q, sometimes significantly. (When you make your own ZynAddSubFX presets, a Q <~ 10 is recommended.)