In reply to:

A real good way to understand this better is to consider your Sub woofers.
Let's say you have 1 Sub rated 500watts RMS at 4ohm.
And you have a 2 channel amp (250 watts x 2 chan @ 4 ohm) to run it.
First bridge the amp, giving the above mentioned amp (500 x 1 chan @ 4 ohm)



Not exactly. You see, when you're bridging an amp, you're adding the voltages of both outputs, hence doubling the output voltage, not the power. Since P=UČ / R, the power output of the amp will quadruple when you double the voltage. So the 2 * 250W amp becomes a 1 * 1000W amp. You'll need a very good amp to reach those figures though. Cheap amps will only double their output because of inferior end transistors, a good (read expensive ) amp will often near the figures.
Take a look here for example:http://www.audison.com/thesis/ethesi2.htm , you'll notice in bridged mode, it goes from 160 to 600W.

So if you want to end up with 500W, use an 8 ohm sub instead (or 2 * 4ohm subs). Because your impedance is twice as high, the dampening factor (is that the correct word? - I'm Dutch speaking) will double (which is a very good thing for a sub since it quantifies the way it can 'control' the conus of the sub)

Frank