I have scanned the responses and there is some solid advice. I am just going to toss out what I did in my Suburban and not trying to talk you into or out of anything.
Being a big music fan when I bought mine it was the first thing I did. I replaced the head unit with one with a JVC 25 amp output I think it was. Bluetooth receiver was a big deal for me as I link to my phone and Pandora to it. I put my money into replacing all 6 speakers in the car with the best I could afford and that would fit in the existing space.
I added a Sound Ordinance power sub. It fits (barely) under the front passenger seat not taking up space as I travel in this one and didn't want a big sub and amp taking up cargo space. I used to teach kayak building classes and would load it full of supplies, so space was important. I was a little concerned about this choice but have never regretted it.
Here is the
newest version. It is not going to rattle windows but it provides plenty of bass for a realistic levels. I listen to a lot of Jazz and Blues which never had strong bass but this can bring it out no problem. But when I listen to newer music that really pushes the bass level I have to turn the bass down a lot from the older classic music.
Not audiophile quality obvious but I huge improvement over stock and in my opinion has a excellent sound. I have no complaints at all with it. It will get louder than I am going sit and listen to it at and the powered sub produces plenty of bass. For what it is worth.....