The wattage on an amplifier is still based on the load. There is nothing to limit the current, the equation v=v^2/r still holds true. All a class b amplifier is are two power transistor connected directly to a dc voltage. The transistors are biased so they are not all the way on or off. One transistor takes car of the top half of the sine wave, the other takes care of the top half of the sine wave. If any transistor is turned completely on, the transistor will draw as much current as possible through the collector and emmitter (provided the leads are shorted + to ground). The only thing limiting it is the beta of the transistor (resistance between the collector/emitter), and the resistor between the collector and the power source. Simple ohms law still holds true, lowering the resistance increases power, it will draw more current from its power supply, as long as your keeping voltage the same.
P=v * i
p=power (watts)
v=voltage
i=current
Its not that it cannot create power from nothing (it can't simple physics), its that there is nothing to limit the current other than a fuse or the transistor blowing out, or the power supply burning up. An amplifier does not create power whatsoever, it just takes the dc source and causes it to oscillate based on the input of the transistor, everything else is ohms law.
The way they increase the power per speaker is to increase the dc voltage going into the collector on the transistor, and finally the speaker. There is actually a power inverter inside the amplifer, if you look inside there is a large circular coil, thats the transformer. The take the dc voltage, make it oscillate through some mosfet transistors (cheap amps usually use something labled irfzXXXX), step up the voltage through the transformer, and convert it back to dc through a bridge rectifier, (usually a couple of things that look like a transistor but are not (usually have diode symbols on them though). You use this equation w=v^2/r and you will see that per load, ie 4 ohm, increaing the voltage increases wattage.
An increased load, ie. lower resistance will make more power, BUT keep in mind that your amp might not be able to take it.