[tonyvdb]

That is a defiant clipping sound, You are probably clipping the input or the output of the amp. You should turn down the receivers sub output and try the scene again if its gone turn up the output of the amp to match the level you had before and see if it re appears. If it does then you know your clipping the output of the amp.

 

[tonyvdb]

lower frequencies (below15Hz) require allot of power to make the cone move. It also requires the cone to move much more than playing higher frequencies. By putting a HPF on befor the amp you will not send any information below the HPF setting and thus dont over work the amp/sub.