Really stupid question to follow. Are speakers built magnetically shielded or is some material put around them or around the inside of the speaker box? I am thinking of building a subwoofer speaker box and would like to know were to find plans. Will my Sony STR-DA4ES 7.1/100 watts per channel be able to drive a subwoofer without adding an amplifier or will i need an amplifier also. How many watts of subwoofer w/o amplifier could be driven with my receiver without problems.