hello people! have a problem with my standard audio system without nav! all faders are set to Central, but the music is definitely up area almost from the front! the rear speakers, including sub plays very quiet! the bass quality from front and rear is very very bad! It seems to me as if he only plays some bass frequenzes and the other not or very bad! what may be the problem? amp? its a is250 year 2006! sorry for my english! Thanks