Roel, you are correct with regard to matching speaker impedance to the amplifier, but only with tube amps. Doesn't apply to the constant voltage type of solid state amp in use today which has zero output impedance (in theory). The limiting factor is the current that the output stages are able to supply. Most designers cut corners to save money, especially if you don't really need the performance. This is why you don't get double the output when going from 4 ohms to 2 ohms. The current limiters take over at high outputs to protect the output transistors.

Bryan