That or use an outboard crossover like this from MTX thats an ebay auction for $30. Nothing from Boss, Autobahn or anything like that unless you enjoy noise being added into your system.
I actually got an idea about how to deal with the volume issue, but I do not have time to play around with this now. The idea is not to control the volume of the device, but to apply the volume control as a custom filter on the audio data stream. It will require changes in the application to make it AU-enabled but once this is done, creation of the appropriate filter should be a piece of cake.
Hey Jirka are you working with QCar in Cocoa?
Audio Hijack Pro.... is perfect for capture the stream audio from any application.
I use this for change the sound. Not equalizer or other strange things, but for Vacuum tube sound.
The audio interface that i prefer and use is the RME FireFace 400... nothing else meet this audio object...
Edirol UA-1EX - It's from Roland (as in Roland professional synthesizers). It's USB so you bypass the so-so quality "Line Out" earphone jack.
Add a AudioControl Epicenter and AudioControl Three.1 plus subwoofer amp and up to 2 other amps for front & rear
The Three.1 gives you a master volume control from the dashboard, as well as subwoofer control, balance and fade.
See my CarPC specs for details