Easy.....take the Monsoon amp out of the loop. Just buy a 200w 4 channel amp. Thats more then good enough to power the stock speakers. You also might consider getting a sub for the system. I have been able to get very nice mid's and high's out of the Empeg - Aftermarket Amp - Stock Speakers setup, but there is no real lows. Another month and this goes in.....
http://www.jlaudio.com/enclosures/81microsub.html