Very few amplifiers/speakers on the market are over/under rated.
If Brand X claims that their amplifier can put out 800 watts, then it can.
What many people don't realize is how manufacturers get these numbers.
Now say I own a big Car audio Manufacturing Company (I don't but this is just to help prove this fact). If I desparately need to sell my amplifier, that normally puts out say 600watts max, how would I go about it.
Well I could say that it puts out more wattage than it really does @ 14.4VDC, but wait, no I can't advertising regulations prohibit it (at least in Canada/USA they do).
So I get to thinking "what if I could make my amp put out this higher wattage? Well then I could advertise it as a higher wattage amp. Now, how do I do this?
I know I'll run it at a HIGHER VOLTAGE. No, that won't work, I'llburn out the amp. Wait!!! What if I FREEZE it, say, with DRY ICE?
The Unit would remain at a much lower temperature, so it could run longer at the higher voltage and not burn out (plus everyone knows higher voltage = higher output).
Using this method I get the amp running at 24V stable and discover that my 600watt amp can put out over 1000 watts. I can now advertise this amp as a 1000 watt amp, and sell it either at a higher price, or at the same price and get more sales."
You see I couldn't be acused of false advertising because the amplifier can put out 1000 watts. But what the people buying the amp don't know is that it only puts out the high power at high voltage levels.
This technique can also be used for speakers/subs. If you freeze the motor assembly (Voice coil, Magnet, & Spider), you could run the speaker at a very high wattage, much higher than at roomer temperature. This is because when you OVERPOWER a speaker the main reason it fails is the voice coil gets burnt out
Many, many companies have gotten away with this. Some of the well known, not necessarily liked, brands that do this are Pyramid, Sony Xplod, AudioVox, and pretty well anything Radio Shack carries. These amps aren't really overrated, just no-one can run them at the 24Volts without them burning up.
Basically go out and get a pyramid amplifier and run it exactly the same way pyramid did when they tested it. You will find that it meets their quoted specs.
Companies like JL, RF, PG, Alpine, Pioneer, and most other TOP NAME BRANDS will give you specs based on either 12Volts or 14.4 Volts, as this is what the equipment will be run with most of the time ( I say most because they are extremists out there that run amps at 18 volts). Thus giving you a more realistic nuber to expect for output.