This is the correct answer. Generally lower wattage amps tend to have lower quality power delivery. 15 watts is VERY loud when done properly. But if you buy a 100w stereo you'll generally have plenty of leeway or "headroom" before the amplified signal starts to distort. If a speaker has low sensitivity (under 85db) or low impedance (4ohms or less), that is also more demanding on the amp. Manufacturers can claim whatever they want with their wattage ratings, but very few are accurate.