Personally I’d work in pairs, the only exception being that you can get away with one sub(woofer) with two tops (full-range speakers) sometimes.
It really depends on what you want. Clearly you get more amplification if you add a sub, but it is amplification aimed at the very low frequencies (max. crossover is 140Hz for the 18 Mackie). Using a sub for those low frequencies will free up the same frequency range from the two tops (as they will receive their signal after the high-pass filter in the sub), so it has some amp power freed up to power the low-mids and mids. So the entire system will be louder as a whole.
That said, the main reason to go to a sub is to get a fuller and lower low frequency result out of your setup. So if you feel you are lacking low end and need more Umphhhh, then adding a sub is a good plan. If you are happy with the balance of your sound and don’t necessarily look for gut-shaking boom-boom, but more for louder and to cover a bigger area, then buying another pair of 15″s or 12″s would be a better option.
Hope that helps some.
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