A speaker is different from an amplifier in the sense that the speaker is a system that converts an electric signal into acoustic sound. They are not comparable. The amplifier can be seen as one of many parts of the speaker - it does not matter if it is placed internal or external.
The amplifier is a simple system which has easily measurable properties. The output signal is an electric voltage which is a function of the input signal, and those signals vary with time. If the output signal can be verified to be sufficiently similar to the input with the specified gain, the amplifier works perfect.
Only 3 things can happen here - noise, frequency response deviation, nonlinear distortion. All of which can be measured, and we know the audible limits of those. Measurements of good amplifiers show performance that exceeds those audible limits, and thus they should sound transparent - no audible difference between input and output. And indeed they do. There are to this date no published controlled listening tests that shows audible difference on good amplifiers working within their intended signal range.
That does not mean all amplifiers are the same. They must have properties adapted to the application, so that there actually is no noise or distortion when driving the speaker it is connected to, at the chosen listening volume. But that is no mystery, and the selection can be made purely based on technical specifications, there really is no need to "listen".
For a speaker, the situation is by magnitudes more complex. The output can be measurably very different from the input signal, yet still acceptable. This represents challenges both in how to measure and how to interpret and understand those measurements. This is where audio gets interesting.
In the hifi-world you often see those systems with quite modest size speakers, and power amplifers that are larger than the speakers. And the amplifiers are believed to have a similar impact on sound as the speakers - they have "punch", "holography", "prat" and what else you can read in typical hifi-reviews. It would be a nice story, was it not for the fact that the whole concept is based on an illusion.