amp/speaker question

so I've got a little marshall 15 watt practice amp, it has a "line out" jack...can that actually connect that to a larger speaker/cab of some sort?
For example, could I go guitar->practice amp->cab using that jack?
or would I basically have to disconnect the red and white wires from my practice amps speaker and connect them to a bigger speaker?
Is this a reasonable to get a louder sound, or should I just shell out 300-400 for an amp with more wattage?