The db scale is a logrithmic scale wherein 3 db represents a doubling of power.
When the pre-amp is set at 0db it's running full-out.
The Marantz MA-9s2 user guide has an excellent chart that I will attempt to illustrate. The power rating is 300w (mono)
Indicated level ( 8ohm load )
All you need to remember is that your Mc is 250w; for every 10db down,move the decimal point one space to the left from its 250w power rating. If you're running 4ohm speakers the wattage would be doubled (with high quality solid-state designs-most amps will qualify this in the specs ie: 300w @8ohms, 600w @4ohms)