i have a 2 ch. Concept CC-D1200a amplifier pushing 2 MA Audio 12" XE competition subs.
a friend said i could bridge the amp to make the subs louder is this true?
if so how to do this?

 

Use the 'search' feature. I'm sure there's a lot of posts on 'bridging.' But if you're too lazy, read on.

A lot of people don't understand the concept of impedance and the amp's ability to handle a certain impedance. So here I go...

Subwoofers are usually either single voice coil or dual voice coil (DVC).
Each coil has a certain amount of resistance (also known as impedance, or more commonly known as OHMs)
The ohm is the amount of resistance between the + and - terminals.
The higher the ohm, the less efficient it is so therefore requires more power.
And the opposite is true, the lower the ohm, the more efficient it is so therefore requires less power. (BOSE sound systems)
Most automotive door speakers are single voice coil and measure 4 ohms.
Most subwoofers vary between single and DVC and also 2, 4, or 8 ohms.
A lot of times, more than one subwoofer is ran together to a single channel. (Bridged)
When bridging two or more subwoofers, all the voice coils must be wired to form either a series or parallel circuit. (Two 4 ohm voice coils in parallel (+ to +, - to -) will result in one voice coil rated at 2 ohms.)(Two 4 ohm voice coils in series is (+ from one coil is connect to - off the other coil and the left over + and - are used to create a voice coil) rated at 8 ohms)
Two channels can be used together to make a single channel (Also known as Bridged)
The lower the ohm (resistance) going to the amp, the more power the amp will put out therefore, twice the ohm will effectively decrease power in half.
Most amps will handle down to 2 ohms per channel but two channels bridged together will only handle down to 4 ohms. (By 'handle' I mean the amp starts putting out more power at lower ohms and it gets hot and will overheat, usually leading to temporary shut down)
Lower the ohms, the more distortion the amp will put out.
Also lower the ohms, the more current the amp will require. Typically requiring upgraded power/gnd cables and fuse.

Some amps, usually Class-D, will reliably handle down to 2 ohms. Some even down to 1 ohm but usually tends to shut down frequently at higher signal inputs.
Most true Class-D amps will put out full power (w/a 5V signal input) with the gain up 25%.

Also, most common subwoofer failure is due to fried voice coils. This is usually from signal clipping. Keep the subwoofer output (on the radio or other device before the amplifier) down. If the amp doesn't get loud enough with the sub volume at 3/4 and the gain all the way up... face it, your amps not strong enough.