I asked the same question not that long ago. This could be slightly wrong but... I was told that 0db is the max output of the amp (its RATED power at least with low distortion), and that anything in the + range, the amp is boosting the signal (and maybe has a few percent distortion rather then 0.03 etc...). At say -20db the amp isnt letting all its power through the signal to the speakers. My Harman Kardon goes from -80 to +10. I normally have about -15 to -10, but have had to 0 for quiet songs, rarely though. You shud be able to hear when it clips and sounds bad though anyway.
Some songs need higher peaks of power however to so there wil be less headroom at higher volumes.