Stock exhaust is tuned for mid-range power band.
Most headers sold move power band to lower range, that seems to be the most popular.
Just FYI, modifying a stock exhaust system never gets you more power, the point of headers is to get the most available power in the RPM range where it will do you the most good, as said, most like low end power, but race cars go for higher RPM power since they spend very little time at lower RPMs, i.e. they don't care about acceleration from 0-60mph, they what the best response from 60-100mph
Mufflers can change the sound, make it louder and deeper.