In most cases, separates offer significant audible performance, especially for two-channel audio, over the best receivers on the market. Power Lets talk about power for a moment. They use this term as a means to an end to boast unrealistic performance of their products under ideal, non-real world conditions. Take a look at power specs from a typical receiver and even some separate multi-channel power amps. Most of these companies rate power in 4 or 2 ohm loads as dynamic.
Dynamic power has little or no meaning, but looks good on paper.
In order to faithfully reproduce the dynamics of music, one must be concerned with continuous power. For example, a bass note doesn't last for a few milliseconds, but can last for several seconds and maybe up to 1 minute. An amp should be able to deliver power on demand both instantaneously and sustained for all frequencies between 20Hz to 20KHz for a reactive, not just a resistive load. The reason being is that if a multi channel amplifier is properly designed with plenty of power output stages for each channel, to handle large amounts of current, then it can deliver far greater power in 2 channel mode if it shares a large common power supply as opposed to smaller individual power supplies dedicated for each channel. A Note About Speakers Speakers are not purely resistive and actually change impedance as a function of frequency.