He's either putting out too much power for the speakers to handle (in which case will eventually blow them) or it's not getting enough juice to pump out the louder volume.
Your amp is probably going into overload protection. Are the sub(s) and door speakers wired to different amps or to a single amp? Are the sub and door speaker wired together off a single channel of an amp?
He's either putting out too much power for the speakers to handle (in which case will eventually blow them) or it's not getting enough juice to pump out the louder volume.