I need to replace a speaker (6.5") on my antiquated Sentra. I'm guessing that the total power output of the HU is 20 watts, if that. What numbers do I look for in a speaker so that I don't buy "too much" speaker for a weak-output HU? I just want to make sure it has enough power to run the speakers (not looking for high volume). I'm looking at about $50/pair I guess, and I plan on getting them at Best Buy or similar type of store..