Turn them off and carefully push in on the cone (gently) and listen for scratching or scraping- push strait in and they should flow freely in and out if they DO without feeling or hearing scraping the subs are probably ok, next its either the amp is not powerful enough or the box is not soundly built, by that i mean its too small for the subs- is the box ported or sealed? if ported try plugging off the vent temporarily and see if bass increases if it does i would permanently seal the box