Well I'm not sure the tablet I purchased will be the last tablet I'm going to use. So I would rather not open it up and hunt/peck for a 12v source to tap into. I figured using a USB port would create a modular style amp trigger circuit. The tablets DC power supply will get ACC current for charging the internal tablet battery. But to prevent any kind of turn-on thump I would rather have the amps kick on when the USB ports are activated on bootup.
usb 5v is monitored and have specific current limits
there are USB controlled relays out there (becarefull though some violate usb design rules for suspend)
there is mine of course... whiuch you can purchase right here a mp3car.com (MJSG USB-IO-1)
might do more than you want...
and is controlled by front end application