Arent those subs, 400 watt rms? if im not mistaken, i know the 12's are at 400 or 500. So you arent getting the full potential out of those subs in the first place.
I'm not to familar with these subs, but correct me if im wrong, a 600 watt amp can't power two 400watt subs (total 800 watt rms)