Preamp Volume Question

I really doubt that your volume is that linear, there's too many other factors involved. The amp's output won't be that straightforward either. You won't find a person here who can state that at 1/2 volume, they are getting 50% of the power output to the speakers...

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 )
0db 300watts
-10db 30w
-20db 3w
-30db .3w
-40db .03w

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)