How is the power range determined?

I agree with the good comments by Stan, Herman, and Mmarvin19. But I'd add the slight qualification that in the case of speakers such as the original poster described (300W power handling, 90db sensitivity), going to a 400W or so amp (that provides good sound quality) will result in a very limited set of choices, which will also be very expensive (apart possibly for some Class D amps).

300W into a 90db speaker represents a sound pressure level of 115db at 1 meter, from each speaker, a situation which is unlikely to lead to clipping. The 30% over mfr. rating rule of thumb strikes me as a good guideline for speakers that have more ordinary (lower) combinations of power handling and sensitivity, and perhaps situations where the room is exceptionally large.

Regards,
-- Al

Good topic. I am of the mindset that, in general, people tend to underpower their systems. Although a certain decibal level may be produced by a speaker, it does not guarantee that it will be able to properly delineate the details. The hallmark of underpowering is not clipping, but a lack of focus when things get congested or trouble with huge dynamic contrasts. These things, to get do them correctly, need a certain power reserve. Perhaps the ratings for lower power on speakers are as much a nod to the market constraints.

With that said, there may be some great amps of lower power that do many things quite well, with certain types of music, in smaller rooms. However, having absolute control of the bass, clarity and soundstaging during congested passages, and the proper speed for dynamic shifts may be the downsides.

You guys are FANTASTIC.
Many thanks for wonderful tips.

Recommended amplification for my current 91db speakers was 25-150W. I figured that 70 tube watts would be plenty; and it was, until I through 200 watts at them. I couldn't believe how much music I was missing. Another benefit is that my system became musically satisfying at much lower volumes than it used to.

From a marketing point of view, it's in the best interest of a manufacturer to claim that his speakers are compatible with the widest range of supporting products possible. If this makes sense, then that low recommended figure will nearly always be a stretch.