Alright, to answer all the questions.
The budget would be $300 to $600 Per sub, i was thinking of a 2 sub setup, so total would be around 1,200. I would wait to get a bigger budget, but we need the punch to get the audience feeling the music. People like bass, and the severe lack of bass we have is disappointing. (Also im a bassist so i have a love for the low frequency)
We are a Screamo Metal band and we play in drop tunings a lot (Drop C and sometimes drop A), so we definitely need the separation of lows from the bass, and kick (Our vocalist doesn't do lows so that isn't an issue i don't think) and the Highs from our mains. However i know i can't run Aux-Fed because for one, the bass has a LOT of high frequencies because of the strings i use (Very punchy sound, hard to explain except with and example like Dig, by Mudvayne) and our mixer is basic and we need the aux sends for monitors (We have 4 Aux sends on a 16 channel analog mixer) Our current mains are each 200w active single 15" speakers. i think the response was like 40hz to 20khz (If that matters) I'm looking to upgrade this to a much more powerful passive system, but the bass is needed to drive more.
We usually play at generally high volumes (and 200w can carry in a small room), at least for small gigs up to about 400 people max and usually play on floor, not a stage. (When it is a stage, its not high enough to place a sub, or the place has a better PA anyway)
If it matters, i have read up that some subs instead of a unidirectional pattern have omni-directional patterns (at least compared to most subs) i would be looking for a more omni-directional enclosure, and basically want to rock the house as much as i can with the budget.
As for ported designs, which are the most effective? I've seen some where the ports run down the sides of the speaker, i think it was an EAW that did that, and probably a few others, i think the most common I've seen is the ported bottom, etc. What effect do different port types have on the sound and volume of the sub?
Hope this clears things up, and thanks for all the help so far!