0dB means no attenuation, ie you're getting all the range of the 16-bit DAC. The volume control is digital, which means that turning it down effectively loses resolution - this is in common with all other digitial volume control solutions (unless, eg, you use 24 bit dacs and only feed them a 16-bit signal and use the other bits for volume control).

As it notes in the empeg manual, you should turn down your amp gain so that 0dB is the loudest you want to hear - this also reduces background noise.

Hugo