Decibels are meaningless without a reference level. For your stereo, the reference is the power at maximum volume. Here, dB=10*log(Pmax/P). When you crank up the volume, P increases until it's equal to Pmax, giving a dB of 0.
I'm not sure how to relate power to apparent sound volume, but my guess is that there is an exponential relationship, P^2 or P^3 or something. In that case, a linear increase in dB will give a linear increase in percieved volume.
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