The wattage is always the same in the head(power),connecting another speaker will mean that there is more air being moved thus making the amp sound louder,the headroom will probably seem alot more but it really all comes down to how much power the amp has,I would definatly run another cab to push some air,
Im sure its like a 12" speaker mover 12 cubic inches per movement of the speaker so if you have a 2x12" its moving 24 cubic inches per movement,It you have that connected to a 2x12" combo then.,....well you get the jist
hope it helps,remeber to connect them properly with the correct ohmage goin to the speakers
Euan