This is a rule of thumb. A 50 watt amp will produce a sound equal to the speakers sensitivity at normal listening levels with some headroom to cover the peaks. Therefor, If your speakers are rated at 86 dB, it will take 50 watts of power to deliver this level of distortion free sound at a listening level of 3 meters or less. To deliver 106 dB under the same conditions, you would need 100 watts. I personally would not use an amp of less than 60 watts with speakers rated at 86 dB...you won't be happy with your system if you do.
HG
HG