Stereo systems & audio setups vary greatly depending on what type of music you listen to.
A good head unit, multiple interior speakers (tweeter, mids, bass) along with a subwoofer in the trunk is pretty much a given no matter your tastes.
Anything additional or replacements you do are usually better than the factory OEM system.
Some people prefer to listen to the motor.
Some like their jams.
Here’s a few trunk layouts that your fellow Buick Regal owners have created.