powersoft m-drive problem

Hi, I have an m-force subwoofer, m-drive amplifier and diaphragm from powersoft and I want to make a 6th order box for it that works in the range of 10 Hz to 80 Hz. A major problem I have is that the power does not increase more than 1200 watts, which also decreases quickly with increasing temperature. Is it possible to use another amplifier like trampes md12000?
And how can I get the most power from this system? Because m-force can work well with 5 kW