Do what Tony says until you get it just right to where the sound is perfect. Then change it back to a flat setting and it'll sound 10x better.

Paul isn't just trying to be funny here.

If there's anything I have learned in tweaking my highly tunable system (8 channel "main" amp with separate gain and bandpass crossover controls on each channel pair; subwoofer amp with 82 selectable crossover points (41 for low pass, 41 for high pass), four frequency centers and 32 level settings for "Q-bass enhancement", continuously adjustable high/full/low pass pass-through outputs; dual 1/3 octave equalizers (one for left channel, one for right channel); remote gain controls for center channel and subwoofers; etc.)... but, I digress.

As I starated to say before I rudely interrupted myself, if there's anything I have learned, if I am tweaking by ear, I cannot do it for more than about 20 minutes at a time. After 20 minutes, anything I change makes it sound better. I'll be convinced that I am a Golden Ears genius -- until the next morning when I get in the car and hear it with fresh ears and wonder just what in the world I could have been thinking.

One option that nobody has mentioned yet... take the car to a high-end stereo shop, and if they are any good they will have a high quality RTA system with a calibrated microphone and somebody who knows how to use it. It will probably cost you $35-$50, but you can have them "flat-line" your system for you -- assuming you have enough tuning controls in your system to do that.

Even if every speaker in your car were "perfect" (there is no such thing) you would still need to tune them so that you can induce "imperfections" designed to compensate for the acoustics of your vehicle and the interactions of the various speakers with each other.

I have found that the only way I can tune a system by ear is to listen to it for at least a day or two, then decide that (for example) there is a resonance in the 350-500 Hz range that needs to be attenuated, and then with the stereo turned off, take 2--3 dB out of the appropriate equalizer band(s). After that, I'll listen for another day or two and then move on to the next problem area. It can take months to tune a system this way.

tanstaafl.