How much power is your line out pushing? You may need an in-line headphone amp.
I went looking for headphone amps with their own EQ, because the BRZ's eq is not all that great, and I found some interesting things. SoundBlaster's E1, E3, and E5 have a computer-configurable EQ program that, among other things, will reduce background noise and amplify dialog. They're designed as portable amps, but it may be worth contacting them and outlining your needs to see if their products can meet your goals. Maybe you can hardwire one into your car.
http://us.creative.com/p/sound-blaster/sound-blaster-e1