I need to know what to buy for my amp?

i got an amp but it doesn't have any place to plug in a bass control and i need something for that so i don't have to keep going back and forth to turn up and to turn it down ive heard about cross overs and equalizers and epicenters would those work? i want to put it between the amp and the sub to control how much wattage is going to the amp so i can turn it up and turn it down