Here, I show the configuration.
The 44.1kHz 16bit S/PDIF signal is converted to 88.2kHz 24bit signal to minimize a quantization error using Behringer SRC2496. It is transferred to the digital equalizer, Behringer DEQ2496, which was installed beneath a driver's seat and fixed by velcro fastening.
A characteristic feature of DEQ2496 is automatic equalization with a measurement microphone. Behringer ECM8000 is inexpensive and suitable one for the measurement of acoustics. I measured a frequency response of my speakers using DEQ2496 and ECM8000.
DEQ2496 can automatically flatten the frequency response by the graphic equalizer. Starting from the automatically set config, the graphic equalizer was manually set for fine tuning. Listening to some music sources, the equalizer was adjusted so that the speakers had little presence in the acoustic field. The following photos show settings of the graphic and parametric equalizers of DEQ2496.
Since high frequency had significantly low response because of the installed position of speakers, it was compensated with another digital equalizer in the digital amplifier.
In practice, the frequency response was roughly flattened using it, and then finely tuned by DEQ2496.
How was the sound with the equalizer? Amazing. Really amazing. Now I understand that the equalization is essential for car audio. With the digital system, I could not recognize any loss of sound quality from the equalizer. The Behringer products, SRC2496, DEQ2496, and ECM8000 are inexpensive but valuable devices not only for studio but also car audio.