The difference between 100 and 200 watts is only 3db, or a just noticable difference in loudness capability. The need for the higher power is if you have inefficient speakers (85db/watt or so), speakers that are particularly hard to drive (they may have impedance that dips down under 4 ohms) or just like to listen at really loud levels. If your speakers are what would be considered "small", 100 watts is probably enough since they would not normally receive low frequencies below about 80Hz, and this is where the high power demands are. You subwoofer's amplifier would handle the band below 80Hz.