How can a 40 watt amp outshine a 140 watt amp

A 40 watt amp can out perform a 140 watt amp in two ways. First, a watt is current into a resistance, and the resistance is the speaker. But the speaker resistance varies with frequency. As the resistance offered by the speaker drops, the amp has to put out more current. If the 40 watt rating is based on 8-ohms, the amp would have to put out 80 watts into 4-ohms and 160 watts into 2-ohms. Only if the 40 watt amp has a capable power supply can that happen. That requires an expensive power supply (bigger transformer, higher quality and capacity of filtering caps). The Pioneer 140 watt may not have that kind of a power supply. It is rated at 140 watts at 8-ohms, but if the speaker load drops to 4-ohms, the amp may not put out 280 watts to drive the load, instead the power supply only puts out, say 200 watts, and at 2-ohms it says "I give up" and just clips.

The second factor is the quality of the output devices. The more expensive transistors are more linear and stable and distort the signal to a far lesser degree. High quality output devices in a smaller watt amp will definitely out perform a mass market higher powered amp.

When comparing amps, it is critical to note the rating at 4-ohms and, if able, 2-ohms in addition to the standard 8-ohms. The ability of an amp to put out current to a varying load maintains its sound quality throughout the audio band. But that is what drives up the price of an amp -- the power supply and output devices.