Blatty or Gated sound -

This is one of those "dead giveaway" symptoms for distortion devices and in fact for most effects. When an effect makes no sound at all until the input is very strong or very weak, something inside is biased improperly. The signal itself is pushing something into the range where it can pass sounds, but only when the signal level is right. There are two exceptions to this - Noise Gates and Compressors. These devices are supposed to gate or modify the amplitudes of the signal  as part of their normal operation. Debugging them is best done by other means.

This happens two ways:

In both cases, the procedure is the same - measure the DC voltages on the transistors and opamps in the effect, and figure out which stage or part is biased wrong.