Amplifier output power is measured in WATTS, and you should only pay attention to the RMS Power Specification of any Amplifier. The amount of watts which you get from an Amplifier depends on the amount of load which you connect to it. The Load is often refered to as impedance and measured in Ohms.

Most amplifiers reach their rated power when driving a 4 ohm load. Some amplifiers on the market will also work down to 2 Ohms, but this is not really relevant, since 8 ohm and 4 ohm loads are more common for Mobile D.J's and 2 Ohm loads are rarely used. So pay more attention to the 8 and 4 Ohm ratings when purchasing an amplifier.

Replacing your existing speakers with higher wattaged ones will NOT increase the power level of your amplifier.

The amplifier will only deliver its 8 ohm rating to the speaker, regardless of whether the speaker is rated at 200W or 600W. The amplifier dictates the power output not the speaker!!.

So for instance if your amp said something like:-

400W RMS @ 4 ohms
270W RMS @ 8 Ohms

Your amp would only deliver 270W into an 8 Ohm speaker regardless of whether the 8 ohm speaker itself was rated at 300W or 800W.

So if your amp will only deliver 270W @ 8 Ohms (as in the example), you won't magically get 800W from the amp simply by connecting a higher RATED 800W speaker.

The only way that you'll get more power from your existing amplifier is by connecting another pair of 8 Ohm speakers alongside your existing speakers. So effectively you'll be using 4 speakers - 2 per channel, which will lower the impedance (load) on your amplifier from 8 ohms per channel to 4 Ohms per channel. Thus giving a slight increase in overall power output.

Again don't be fooled into thinking that it'll double your volume, because it won't , sadly the human ear doesn't work like that and as i've said before, in real terms 500W does NOT sound exactly twice as loud as 250W. Yes It will sound louder, but not twice as loud .

If you find that you are using one pair of 8 ohm speakers on an amplifier and it is not really loud enough, then you can increase the power slightly by adding another pair of 8 ohm loudspeakers. However if you are already using 2x 8 ohm speakers per channel, and you are clipping your amplifier on a regular basis, then you should really consider purchasing a larger wattaged system.