In reply to:
dB is a relative unit, and has no meaning without a reference level. In the case of an analogue signal, we could state that our reference was eg 1V, and then -3dB would be 0.5V, and +3dB would be 1.5V. In the digital domain, once all of the bits are 1, you cannot get any higher, so if you did state that this level was your 0dB reference point, then only negative dB values make sense, and any signal that attempted to exceed this value would indeed clip, as you have alluded to.
I haven't read the DSP specs or anything, but having positive dB values still makes sense to me. The dB value represents the gain (the output amplitude relative to the input amplitude) of a given stage (e.g. the volume control). 0dB is the point at which the signal is neither attenuated or amplified, so that if the input signal uses the full digital range (i.e. goes to all bits 1) then it will pass through cleanly. You can add in amplification (i.e. +ve dB values) and clipping will occur if the generated output signal exceeds the full digital range. e.g. If you double the amplitude of the signal, clipping will occur on any input signal that uses more than 50% of the full range. For signals smaller than this, it will be fine.
If you use only attenuation at all stages (volume, eq, etc) then even an input signal that uses the full range will not be clipped, hence the advice to eq using attenuation (-ve dB values).
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