Hi, iv'e seen something very strange for the first time. Iv'e installed by sound from old car to new. A 2000w sub on a 2500w amp. 1 x hi to low convertor and speakers still run of factory radio. Amp mounted on rear seat. When the volume is all the way up and the trunk is open every thing seems fine. But when the trunk is closed and the volume turned softer until 0, the sub keeps on pounding, very loudly. But if the 1500w amp is tested, everything is normal. Could this be because of the hi to low convertor or because the speakers are still running on factory radio?