Others may have other ideas but your BEST bet is to hook a digital processor to your SPDIF connection. This will allow the cleanest sound, no chance for electrical inteferences due to ground loop issues etc.
There are some (Expensive) stand alone units available that are all inclusive and won't require anything outside of what is included. These will have an external keypad or other means of adjusting settings etc. These may be hard to find that will work in the car however. I know Alpine used to have a unit that I believe you controlled through a simple touch screen on the unit for simple stuff or had advanced controls through a supported deck. You may find something similar.
Some on here have used DAC units that will take the SPDIF output and do all sorts of things with it. You would want one you could adjust through a USB port but will run fine without the USB connected. Then you could use a laptop (Or a LONG cable with your desktop) and adjust the DAC settings but still run it fine with your unit.
If you haven't picked up any amps yet you could find a 5 channel amp that takes a pair of high level inputs and takes care of the "Mess" for you... In the long run might be your best bet. And I wouldn't be surprised in this day and age if you could find one with an SPDIF input as well.
But I think you will find most will suggest going the DAC route even if you can't adjust settings with your device and have to use a laptop to do it. You will generally find that you will likely not change the settings once they are initially set. Just have to make sure it will run without the PC attached.
I would be shocked if you can find a DAC than can be controlled by an Android device directly otherwise.