Car audio is a different beast. Perhaps at the high end of competition systems, you paid attention to the size of the car. I wouldn't know, I never got into it that far. I stopped at the lower end of high end car audio. Most vehicles have roughly the same size of interiors, at least within a few cubic feet of one another. Ditto listening distances. So that never entered into the equation. Speaker placement was relatively fixed, depending on the vehicle. Phasing and imaging you dealt with using parametric EQs (these days, higher end head units have that all built in) And you didn't really have "mains" in car audio, at least not in the way home audio thinks of them. So you chose a set of tweeters and mids, usually they came paired with crossovers. 4 door vehicles you used two sets. You used the manufacturers freq. response graphs as a starting point to determine which frequencies you were going to send to the tweeters, mids, high bass, mid bass and low bass drivers. Every car audio speaker has a sweet freq response range that it likes to live in, given the enclosure you were forced to deal with (usually the inside of a car door) And you put the drivers wherever you had the room for them, counting on the parametric and/or head unit to fix imaging issues that would crop up from say, having a tweeter in the A-pillar, a 6.5" mid driver in the mid door area, an 8" driver under the seat, a couple 12s behind the rear seat and a 15" stuffed whereever you had room.
So I always treated my home systems similar. Your question has made me do some digging and I see my assumption was wrong, with 6.5" drivers in good home speakers able to accurately reproduce music way down low, some say as low as 30Hz (I have a hard time believing that number) with a good bookshelf speaker.
So perhaps I don't even need to worry about a sub. My days of wanting to shake the foundations are long gone. Now I want accurate reproduction of my source, tight bass, and no harshness or brightness anywhere in the freq range. I listen to almost every form of music, except hip hop and in almost equal measure. Perhaps a little more blues and 80s rock than the rest. I have limited space I'm willing to to give over and my speaker placement options are limited to either side of my 5 foot wide entertainment unit and a 15" x 15" x 18" corner beside my couch for a sub (if its even necessary).
