Increasing power is the least efficient way of increasing output. Output is going to be dependent on the sensitivity of your subs, overall displacement of your subs and enclosure and lastly power. Its also important the amp is seeing clean power and thus putting out clean power to your subs. Look for an amp that is going to do rated power, make sure your electrical can handle the current draw it will be receiving from the amp (factoring in efficiency) and have it set up properly. If all three parts of your sub stage are correct and you still want more output, then you'll need to restart the cycle.