VW, Memphis Amp getting hot

As far as im aware most/all amps these days have DC protection, but the situation you are describing sounds just like that to me. Any DC offset present within the audio signal will cause the voice coils to heat up rather than producing noise, leading to failure as you described. It would also cause the amps to get hot. If you can, test the output of the head unit to see if there is any DC voltage present within the output signal. Seeing as this is the only component that didnt get replaced it could have been the culprit. Also double check that you arent somehow short circuiting the output of the amp as this would lead to the same overheating problem. The suggestions i have made are unlikely situations, but are the only ones i can think of! As aznboi3644 suggested, turning the volume up too much will fry your sub, but thats so obvious its almost insulting !