Do you have a decibel meter (like a Radio Shack)?
If you do, turn up the volume to your normal listening level at the listening position, then measure the decibel level from a distance of 1 meter directly in front of each speaker (left then right, or vice versa).
This will provide you with the decibel level at 1 meter when the volume is at your normal listening level. Try to measure a couple of volume peaks as well to get an idea of decibel level at loud points in the music.
For your speakers, 1 watt of power produces 90dB measured at 1 meter. Each doubling of power will produce an additional 3dB. So...2 watts will produce 93dB, 4 watts will produce 96dB, 8 watts will produce 99dB and so forth.
Using this info, you can determine how much power you will require to produce the volumes you prefer.
If you do, turn up the volume to your normal listening level at the listening position, then measure the decibel level from a distance of 1 meter directly in front of each speaker (left then right, or vice versa).
This will provide you with the decibel level at 1 meter when the volume is at your normal listening level. Try to measure a couple of volume peaks as well to get an idea of decibel level at loud points in the music.
For your speakers, 1 watt of power produces 90dB measured at 1 meter. Each doubling of power will produce an additional 3dB. So...2 watts will produce 93dB, 4 watts will produce 96dB, 8 watts will produce 99dB and so forth.
Using this info, you can determine how much power you will require to produce the volumes you prefer.