How is the power range determined?

The manufacturer determines the maximum power rating based on how much power the low frequency driver elements (the woofer or woofers) are rated to handle. The high frequency drivers will invariably have much lower power ratings, but music typically contains much less energy at high frequencies than at low frequencies.

The manufacturer's recommended minimum power rating is based on a rough guess as to how much power would be required to play music at reasonable volume in a relatively small room, at a relatively small listening distance.

As a goal, it's generally a good idea to have the power rating of the amplifier around the same as the speaker's recommended maximum power, especially as here if price and size don't matter, and we assume that "everything else equals" (meaning sound quality in particular).

You can get away with less power if your room is not particularly big, your listening distance is not particularly great, and the music you listen to does not have wide dynamic range. Recordings of classical symphony orchestra tend to have the widest dynamic range, meaning that they have the greatest difference in volume between loud and soft passages. Setting the volume level for good reproduction of the soft passages on that kind of material leads to very high power levels being required when the occasional orchestral peaks, bass drum beats, etc., occur.

Regards,
-- Al

Many manufacturers of high end equipment don't give power ratings. The reason is simple...there are many different ways to calculate the power rating so it is a meaningless number to most people. Unless you drive the amp into clipping it is unlikely you will damage the speaker before you reach an uncomfortable volume level. It is far easier to damage a driver with a low power amp driven into clipping than it is with a much higher power amp that only occasionally hits peaks beyond the average rating of the speaker. For this reason you should choose an amp that exceeds, not equals the maximum rating if it is known.

There are different methods to get an idea of how much power you need which depend on a lot of factors, but here is one. It is an oversimplification of the complete picture but gives you a starting point. First estimate how loudly you want to listen to your 90dB speakers and double the 90/dB at 1W for every 3dB. Say you are a metal head who wants to hit 105dB peaks. You need to double it 5 times so 1x2x2x2x2x2 = 32W. That is at 1 meter but you need to quadruple the power every time you double the distance so if you are sitting 4 meters away you need 32x4x4= 512W.

You should go even higher because you don't want the amp to clip. Of course if you sit closer or don't need to hit 105dB peaks your needs go way down.

While it is true that there is generally less energy at higher frequencies UNCLIPPED that is radically changed when the amp clips as it produces distortions and energy at much higher frequencies than the fundamental. Having repaired a large number of PA and sound reinforcement speakers I guarantee you it is almost always the tweeter that is blown and very seldom a woofer.

The bottom line is this....too little is far worse than more than enough.

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.