I'm a gigging jazz drummer and just cause you mentioned "with the back seats down", I gotta say: I always wish there was a car that was optimized / defaulted to having the back seats down. But in a pinch, on those rare occasions where you actually have more than two people in your car, you could pop some seats up.

Every car I get (currently have an Hyundai Elantra GT), I end up with the back seats down 75% of the time, and it just looks crappy. If the default mode was back-seats-down, with no big seams or gaps or unlevel parts, I'd be in heaven.

I run community drumming workshops where each participant gets a single kit drum (snare, tom or bass) strapped to them. We strip the resonant heads and mounting hardware off the drums so they nest inside each other. My Jazz (Fit) will hold enough drums to run a medium sized workshop: five bass drums, five floor toms, five rack toms, five snares, and a bag of sticks/straps/cowbells/blocks/other misc equipment. It's truly a Tardis of a car :)