batteries, amps, speakers vs effect and play time

The amp6 is only 2x15Wrms as any other class D amp on a 12V supply playing into 4 ohms will be.

Power output depends on 3 factors: Supply voltage, speaker impedance and amplifier efficiency. Nothing else.

That also means that each driver only sees 7.5Wrms. Is it possible to damage the amplifier by this? No. It has built in protection, and frankly if a person can't hear severe clipping distortion and turn down volume way before that would become a problem he should probably not listen to music at all as he will have permanent hearing loss.

Technically though with music signals the average is far less. The Boominator for example uses about 4.2W at max volume with music. And 0.7W of those is quiescent current use which is not the same a idle consumption, it's the current used for operations that is unrelated to variations in output power. Idle consumption includes various bias currents as well. So in fact each woofer only sees 0.8W on average at full volume and each piezo sees about 0.05W (the resistor in series sees the other 0.05W).