I’ve been around this hobby for a good while. I’ve had a very “extreme” build for a long time and I’ve had a system that sounds quite good and gets stupid loud, if I say so myself. I’ve also seen and heard many incredible vehicles running the gamut from extreme to rather stock installs. Some installs are easier to tune “out of the box” than others. Some offer things others can’t. But, for the most part, what I’ve experienced is that there has really never been a single defining characteristic to what makes one system sound fantastic other than a) a good install, b) an understanding of the equipment and limitations/implications of the gear/install and c) a good tune based on a & b.
When I think back to my favorite car audio systems as a whole package (sound + aesthetic), the cars that come to mind for me are: Kirk Proffitt’s Acura TL, Jason Bertholomey’s BRZ and Michael Myers' Scion. None have "extreme" builds. Myers has dash pods but it's nothing crazy. All have/had door mounted midbass and a (relatively) understated yet elegant install. There’s nothing that stands out, cosmetically, in a way that draws attention to itself. Yet, they all sound incredible. They just have great tonality and an incredibly enjoyable fun factor. It's like you get in and the speakers just get out of the way and you enjoy the system for what it is. I've done the waaaaay extreme thing: 10 inch midbass in the kicks with a 4 inch hole cut in the firewall, and 5 inch coaxials on the dash. And as great as it sounded, if I’m being completely honest with myself, I preferred what those cars had to offer as an overall package. Don’t get me wrong; I loved my old system. And it wasn’t terribly obtrusive, but for me, personally, there’s really something about having a system that sounds fantastic without seeing what you’re hearing. And while I’m at it, let me be clear that I’m not dissing others’ choices of what they do in their systems. I did things in my old civic that I’m sure others wouldn’t have cared for either. I’m just stating my personal likes/dislikes from my previous builds and drawing inspiration from those I have enjoyed that kept a more stock-like appearance up front.