Probably will be a little quieter since it won't be double amped, but if your stereo has a loud setting (usually Pioneer) You can boost the sound a little louder, or you may just have to turn your radio louder to achieve the same loudness as before... but it will be better for your radio, if you don't think it's getting loud enough, well that would be when it's time for a proper 4 channel, and not trying to use the factory amp to push the after market speakers.... Granted I have a factory deck and factory amp, pushing after market pioneer 4-way speakers, if I did get a HU though, you can be sure I would bypass my amp in a heartbeat. My factory amp now an then seems like the one side of the car fades lower then the other side, or maybe it's just my ears... idk... which is once in a long time anyways when it happens... I think it's just bad practice to integrate with ford factory amps, half the time there is either bad connections, or amps blown if there not bypassed, which is a pain in the ass when a customer comes back and you have to work on their car.... again.... or if they wanted their factory radio put back in and the amp is blown... well... then you get to look into how much a new amp is... and if it's over $200 your ass gets written up, too many write ups.... bye bye.... so, you can see why I would be more concerned than most about how systems are setup.... anyways.... glad I was able to help...
Peace out!