the amp will "work" just as hard either way. It can only put out so many watts and it doesn't care if a certain level is reached when the control is at 9:00 or 12:00. It will run out of steam when that input level pushes the amp to the maximum power the amp can put out, no matter where that appears on the volume control.
And, yes, an eq is best used in the subtractive fashion. A LITTLE boost here and there isn't too bad but too many people use them to boost the bass in order to make their speakers sound, well, bigger which is not it's intended use. Theyare made to compensate for room problems more than speaker deficiencies.