There are a number of things to consider when choosing an amp for your speakers.
1. 100w amp from brand A is not identical to 100w from brand B.
2. To protect the tweeter against clipping (which you may not hear) higher wattage amps are desirable. You will hear clipping on the bass driver so no need to worry about the bass driver.
3. The type of music you listen to will determine how much headroom you need in the Amp. For instance if the music has a large dynamic range I.e very quiet violin followed by loud clashing drums and symbols, you need to increase the volume to hear the violin but don't want the amp to top out when the drums kickin. Classic, Dubstep and Rock tend to have larger dynamic ranges than pop. Also excitable bass heavy music tends to require extra reserves of power from the transformer and capacitors in the amplifier to avoid power dips, larger wattage amps have bigger capacitors and transformers.
4. How loud you want to listen to the music, is a factor. However, less of a factor than you would think. It might take your speakers a 40w amp to produce 80db of sound, but a 120w amp to produce 90db of sound. to increase the sound level requires a multiplier on the amplifier wattage.
5. Your speakers. Some speakers are more sensitive than others. The more sensitive your speakers are the less current they require to produce a loud sound. Also the placement of the speaker in the room can amplify bass notes. For instance placing your speakers 14cm from the corner of room will amplify the bass notes.
The impudence of the speaker will play a large role in the type of amplifier you can choose. I have 8ohm speakers dropping down to 3.8ohm. So I need an amp that is rated at 8ohm but is happy pushing 4ohm.
I have a friend who has 4ohm speakers, that drop down to less than 3ohm. So his amp requirement are different from mine. My amp would struggle with the low impedence for long periods of time.
6. Room type. There are a number of factors with regards to your room type, size, hardness and shape. Shape is the most difficult to deal with so most people don't bother (lots of complex maths). But size and hardness are fairly easy. The larger the room the more power is required to fill it, small rooms do not require as much power. On the other hand rooms with lots of soft furniture, carpet and other items that will absorb the sound waves sound making it sound quieter, thus you may want to increase the volume, which requires more power.
Summary:
For a set of bookshelf/stand mounted speaker you would probably be okay with a 40w/40w in 8 ohm 60w/60w into 6ohm for moderate listening volumes.