it could be coming from many places , the amplifier could be the source of the poping , but i would say the speaker is more likely the source . chances are that you are overdriving the speaker or the amplifier . back the volume off a bit when you notice this . any time you have audible distortion you are approaching damage and even the speaker is rated for more than the amplifier is capable of delivering , it is still possible to overdrive the amplifier or the speaker . but if you are not satified with simply backing down the volume when this happens , i would beleve to be worth while to cautiously try to investigate the source of this , but i would guess that you just simply pushing your system to its limit .