Of course, even if the impedence of the amp and speakers are matched, the efficiency of the speakers (and the sensitivity of your ears) will determine how the volume is adjusted and hence how much power is flowing through the speakers. It is possible to determine the power flowing through the speakers by using an oscilloscope or a multimeter, but that is overkill, just be sensible!

The premise that if you halve the impedence you will double the power output of the amplifier is not stricly true, otherwise logically the amp could deliver 200 watts into 2 ohms. Amplifiers will have a maximum power output into a specified impedence. With a valve amp, the output transformer may be tapped for different impedences and the power at each impedence is the same. It is imperative that the correct total impedence of the speakers (and dummy loads) matches this. For all but the very oldest of transistor amps, an output transformer is not usually part of the design and therefore the impedence of the speakers (which should be matched to maintain a balanced stereo image) can very within a tolerance which is typically 4 to 16 ohms.