I find that when I have passengers in my car, I need to increase the bass and loudness a bit. Dunno why, something about the acoustics.


Well loudness basically increases both bass and treble at low volumes, flattening out as the volume increases. I'm just guessing, but do you use a lower volume when you have passengers in the car than you would otherwise? If so, then try leaving your loudness at the 'passenger' setting and see how it affects the 'lone' sound at 'lone' volumes levels...or try to find a compromise.

But yes, the number of people in a car changes the acoustics. People are basically 80% H2O, the acoustic properties of which are fairly well known to submariners and anyone who ever stuck their head underwater in a bath...it pretty much absorbs most bass frequencies..
When I used to do sound engineering at Uni gigs, I discovered that most professional sound engineers start by eq'ing the room flat, and then pre-compensate for the expected mass of bodies on top of that eq. The result is generally crappy and painful soundchecks.