The only Alpine model I know of that switches to amber is the 7894. I know their are others, but I don't know their models, as I haven't been shopping for a new one lately.
My 7894 is not connected to the battery, and it still draws ample power. Even without a sub in the car, it has sufficeint draw to dim the lights. Also, you are correct about power. The HU is sending a signal to the amp, and the amp is sending power to the speakers. You will still gain SQ, but not as much as if you were to bypass the amp.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>