pre amp volume

I have a sunfire GTP III and sunfire Symphonic Ref. 250 watt amplifier driving Vandersteen 2Cis, used only for stereo so far.

Can someone explain the idea behind the 0 decible volume contol, instead of the usual old style 0-10.

the 2Cis arent very efficient at 88 db
I usually listen at -32 db +/-, subj to the recording,
and -24 dbs is about as loud as I can listen to.
what power is being used?, headroom left? spls?

thanks
rms