Re: 4 channels on Factory Amp
US SLKs all use the Bose system. There are two channels from the head unit to the amp. These are 4-5 volt balanced signals, different from the common RCA 2 volt unbalanced signals of most aftermarket systems. The amp has four output channels. A left and right set of outputs runs to the doors, where there is a tweeter/midrange pair in each side. Two more channels run to the rear, where there is a woofer/sub behind each seat. The rear speakers are bigger than the grill suggests. They are actually aimed backwards and at an angle. Bose typically uses active equalization to compensate for speaker and cabin audio characteristics, so the front and rear drivers are getting different signals. I believe this equalization is built into the amp. The US head units are typically sourced from Alpine. There is no fader control, but there is a balance control.