Bryan, first a little bit of terminology/technology. The volume control on an amplifier, including a sub amplifier, isn't actually a gain control. Amplifiers provide the full designed gain of the incoming voltage whenever they're on and receiving some input. This is typically about 25-30dB of voltage gain, which is an increase in the voltage of a bit less than 30 times. What the volume control does doesn't change the amount of gain, but either allows all the incoming voltage through to be amplified(at its maximum setting)or reduces the incoming voltage to practically zero(at its minimum setting).
Now, at the minimum setting the sound level would be essentially zero, while at the maximum setting when a strong bass signal was input the sub might play at well over 100dB if the sub amplifier was capable of the required output and the sub driver could handle it(neither may be possible; just because there's room left on the volume control doesn't guarantee results). Maybe 40dB over the 75dB figure might be possible with a good sub.
