Unless you have a very benign flat impedance speaker (rare) that doesn't
wander to much from being a flat impedance between 20hz to 20khz, only
then is an amp with watts but little current OK.
This statement is false.
The impedance *curve* is not so important. The *overall* impedance is, and more importantly than that the intention of the designer of the loudspeaker. As an example, many ESLs have a fairly low impedance at high frequencies and yet amps with a fairly high output impedance (aka 'low current') can do quite well on them. This has to do with the simple fact that an ESL is fundamentally different from box speakers in that its impedance curve is not also a map of its efficiency. In a box speaker the impedance peak in the bass is due to the resonance of the driver in the box; its much more efficient at this point. ESLs don't behave this way at all and as a result many solid state amps have troubles making enough power into the bass region of the electrostatic.
This is only one example and there are many more, but the bottom line is that you don't need a flat impedance curve, just an impedance that is high enough.
Of course: ... SETs. They would not exist if the above quote were true.
Of course no speaker has flat frequency response! Trying to get flat response is a Sisyphean task.
