I can´t help you direct, but also take a look at the BXSCR SR1 high-end portable loudspeaker system, it has a fast car charger
integrated (10-18VDC) for powering and charging the internal battery. The car-cable needed costs 29 euro (with sigarette lighter plug).

English product information will be added soon, but look at the extremely low distortion figure of the SR1 with -102dB / 0.0008 % at rated output, also a digital-input version is available with integrated 24-bit effective resolution DAC (total THD in digital mode
typical -100 dB/ 0.0010 %)

greetings Bart