Hi SatKartr,
Getting good car audio is kind of a nightmare, given the huge variability of car interiors and speaker locations. I used to serve as a judge at a car audio competition held in Toronto called "Northern Thunder," and I heard some very good custom installations as well as some really awful ones that just played at absurdly loud levels with ridiculous bass.
All that said, I don't know how old you are, but as I've grown older I find the intrusion of wind, engine, and road noise on top of relatively loud music playback (to overcome the interior noise) quite tiring at highway speeds (about 75 mph or 120 kph).
Those factors may account for some of what you are experiencing. I used a RatShack SPL meter to measure the interior SPL levels at hiway speeds and was shocked that the levels were over 100 dB SPL combined road noise and music, with subwoofer at 75 mph.
I cut back a lot for fear of damaging my hearing. The answer, as you mention, may simply be a car with much better insulation against road, wind and engine noise.
Regards,
Alan
