How is the power range determined?

For the full range speaker forget about sensitivity at all since it's being only measured @1000hz. Other frequency ranges will have certainly different parameters. The only thing to look for is the minimal power rating where more-likely your amp will clip at desirable volume so in your case I'd look first for the amps >50W and than depending on your listening tastes and room I would adjust it further and finally I will choose the one with sufficient power, affordable price and good sound to my ears.

For the full range speaker forget about sensitivity at all since it's being only measured @1000hz.

While the last part may be true (it is only measured at a certain frequency) I must disagree with the advice to ignore the sensitivity rating. It is true that sensitivity varies with frequency but if it is so grossly different that it renders nominal sensitivity irrelevant then you should be looking for a speaker with a flatter frequency response.

My rule of thumb with "normal" speakers (6 to 8 ohms, 86dB to 91dB)is to use an amp with 30% more watts over what the mnfc recommends. Dali recommends a max of 150w for my Ikon6 towers. I drive them with a 2 x 200w Aragon 4004 MKII. There is no doubt in my mind that the Ikons could easily take 300wpc if played at reasonable volume.

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.