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What is the recommended speaker wattage rating (6.5" / 3 way) for a 4x45 watt RMS car stereo?

I am installing a new Sony car stereo and want to upgrade the existing speakers, I want to get the best compatible speakers for the stereo's wattage rating that will give me the best sound performance without overloading the speakers or having insufficient power to the speakers for sufficient volume/sound reproduction.