How is the power range determined?

Go for the amp whose sound you like the best and still has sufficient power. Herman is right about erring on the high side. Al has given you good advice also. Just remember that because 2 amps have the same power rating does not mean that they will sound the same. Try as many amps as you can, you will be surprised at the differences.

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