The decibel level is a meaure of the "gain" of the pre-amplifier stage of the empeg audio circuit when compared to the units maximum potential output power.

Therefore, a gain of 0dB represents no reduction of output power (play a full volume track and get a full 4V output). A gain of -3dB would be half the theoretical maximum output, -6dB would be a quater etc.
-58dB is generally regarded as no output for most pre-amplifier circuits (almost completely silent) and of course a "minus infinity" dB gain is completely silent.