Class D or A/B or Class T is about the circuitry and efficiency of the amps. Normally Class D is above Class AB but does not always mean better sound than Class AB. Class T offers better compromise in between Class AB and Class D. But I am not much aware of Class T amps in cars. As far as Battery drain is concerned there are ways to make sure that the amp is turned off with the car key. (eg..: Cigarette lighter wire). Do search on forum. However this is not good since the amp turns on with car ignition, when one may not want to use ICE at all for certain drives, which means power waste. Your choice for Active Sub is great, but it is a popular belief that they lack the requisite punch (power) and people generally tend to go for custom box etc. As regards Active portable subs, the size of woofer is very important. The bigger the better, anyday. The surface area matters for lows therefore subs are made with larger wafers. There is a reason tweeters too can do their job in such a small package. There is nothing to debate here except price vs performance. Agreed a 12" compact sub enclosure may not be as well designed as an 8" for example, but that is because of design and manufacturing limitations only. I would recommend you to buy minimum 10" and above for active compact enclosure. Even if it means stretching your budget slightly. The serious SQ bottleneck in our Puntos and Lineas is the limitation of amps that can be used for plug and play and the DAC and filters used for sound processing and output. Thereby making the factory based system a pain to upgrade. The best sound installer will be limited by these inherently limiting factory fitted players.