How Much Power is Too Much Power?
This is one that I hear a lot, and unfortunately a lot of the time the answers people give are crap. This question ultimately stems from a misunderstanding of how an amplifier works. Most of the people that ask this question are under the impression that an amp will put out its rated power all the time, so for example a 500 watt mono amp is ALWAYS feeding your sub 500 watts, regardless of what you're listening to or at what volume, and if your sub can't handle 500 watts it will blow.
All it takes is a little reasoning to see this can't be the case. All a speaker does is turn electrical energy into thermal energy (heat) and acoustic energy (sound). If you turn down the volume, the speaker gets quieter and cooler right? So if the amp is still putting out 500 watts, where is that extra power going? The answer is simple, it's not putting out 500 watts, when you turn down the volume the amplifier's output drops. This isn't just limited to volume setting either, it directly depends on what you're playing as well. The simplest way to think about it is this: the louder the sound is, the more power you're asking of your amplifier.
That said, let's go back to the question at hand, how much power is too much power? Well as we just went through, the amount of power an amp is actually putting out depends directly on what you're playing and how loud you're playing it. In other words, if you're playing the Backstreet Boys at 90dB, it doesn't matter whether you're amp is capable of 10 watts, 100 watts, 1,000 watts, or 1,000,000 watts, the speakers are getting the exact same amount of power and are under the exact same amount of stress no matter what amp is powering them (so long as it's not clipping). So, how much power is too much power? There's no real answer, because there's no such thing as too much power. We're not talking about how much power you're actually sending your speakers, of course there's an upper limit there, we're talking about how much power you have available to send to your speakers if the demand is there (the "demand" being controlled by you and the volume knob). Saying there's no such thing as having too much power is just like saying there's no such thing as having too much beer in the fridge. Nobody's saying you're going to go drink all the beer in one night, it's just there waiting for you in case you want it, an amplifier is the same way.
The question really isn't "how much power is too much", it's "how much power is enough", and unfortunately I can't answer that for you, nobody can, because everybody is different. Most of the time the power ratings given to speakers are less of an absolute thermal/mechanical limit and more of a recommended power. In other words, the number they put there is like a suggestion, it's high enough to get the most out of the speakers without clipping the amp, and low enough so you don't have a lot of wasted power (buying so much amplifier that you never really use it to its full ability). Because of that many people like to match or just slightly exceed the speaker's rating, and that's just fine. If you choose not to go that path that's fine too, I personally never want to worry about clipping, in every one of my setups I want the first sign of distortion to be from the speakers, not the amps. I want to be able to get all I possibly can out of the speakers no matter how my filters are set or what I'm listening to. Because of that, I tend to buy very large amplifiers, most of the speakers in my car and home are being powered by amps rated at 2-4x the speakers' ratings. Again there's nothing wrong with this, and I've never had a speaker blow, it's just personal preference. You need to decide for yourself where this point of diminishing returns lies for you.