When i was around 15 years old I built a sealed box without a seperation piece between the subs and they fought each other just like this. The enclosure may have an air leak where the subs a seperated. When i put a seperator between the subs, the problem was solved.
type s subs are 500 rms, they can handle the 500 rms but at that level he was playing, after having owned one, they dont handle more than 500 rms very well without super heating >.