2 ohm or 4 ohm L5?
Mono sub amp, 2 ohm stable (or less): you can use either a 2 or 4 ohm sub
Stereo amp, 2 ohm stable, bridged: Get the 4 ohm
Stereo amp, 1 ohm stable (or less), bridged: The 2 or the 4 will work
Stereo amp, running tri mode: get the 4 ohm
The lower the load, the more power (theoretically) your amp will provide, but keep in mind the lower the load (lower resistance) of the sub, the hotter your amp will run.
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Be aware that using lower resistance loads and bridging produces a greater load on the amp. Well designed amps that cannot handle the demand will either shut themselves off or blow a fuse. A poorer designed amp can permanently damage itself. Also, even if an amp works in these configurations it will probably generate more heat so ventilation is even more important.
As mentioned earlier ohms are a measure of electrical resistance. You should be able to understand why resistance changes affect the amp power as it does. If you raise the resistance the amp is not able to drive as much current through the load and thus you get less power. If you lower the resistance of the load math says that more current will be drawn from the amp. Assuming the amp can handle this you get more power.
Also remember a stereo amp which is 2 ohm stable (each channel is 2 ohm stable) is only 4 ohm stable when bridged.


