I need to replace my entertainment center

So when adding an amp to a car stereo, the speakers connected to the amp and the speakers connected directly to the stereo all work at the same time? I didn't know that.

yes the speaker connections on the head unit and auxiliary outputs for external amps will work at the same time.
you can install a switch to the added amplifiers turn on wire that then connects to the power antenna wire-(amp turn on lead)on the head unit to turn off the speakers you have connected to them but still be able to run the speakers connected to the head unit normally and every thing will turn off when you turn off the head unit.