It sounds like the woofers may have already been damaged by your playing them louder than they are capable of playing.
If the 8 inch woofers are not damaged you would need to remove low bass from them to get them to play louder. If you have tone controls turn the bass down. If you have a low filter turn it on. You could try building a 30-50hz high pass filter for the sub but that would also reduce the deep bass and suck up some power too.
You could also replace the woofers with units that handle more power and will play louder.