Have dual 12" 400 watt rms pioneer subs in pioneer box, wired down to 2 ohm as they're powered by a 1000 watt rms alpine monoblock. when at low volume levels they are fine, but when the volume is turned up (not rediculously far) they start to make a 'knocking' sound when they push out. it doesn't look to me like they're extending far enough to be on the edge of blowing and like i said the volume only has to be on maybe half to make this happen.... wat can i do? cheers henbo