Does anyone know the exact difference between 2ohm and 4 ohm.....

 

It changes teh impedences the amp sees. The lower the impedence, the more power the sub is given from the amp. Hence a 2 ohm subwoofer will recieve more power from the same number of channels on an amp.

 

THat is true that when you lower the load to the amplifier (i.e. attaching a 2 ohm instead of a 4 ohm sub) you do increase power (unless you have a JL amp of course ). Now this comes with the consequence of decreasing stability. Your amp also gets hotter, distortion increases, damping factor decreases. Thankfully for subs, most of these issues are not a huge deal.

Now the actual physical difference has to do with how the voice coil is wound. A 2-ohm and 4-ohm sub from the same manufacturer can have slightly different theile/small parameters, but for the most part it's negligible.