if its a 2 channel you cant add another sub (assuming your current sub is a dual 2 ohm to have it bridged right now) because two subs would drop your impeadence in half and your amp wouldnt be able to take it. you could buy another and wire them both to 4 ohms and then run them on seperate channels but then youd only get 100 wrms to each sub, and it would end up around the same volume as one of those subs bridged (or less)
you can get some great subs for 300, but i dont think many will "pound ***" with that amp.
i think your amp is whats holding you back right now. it might be worth it to spend only a couple hundred on subs, save a hundred towards your amp, and then save up a little more to buy the amp.
also, JL audio is a GREAT company... but it sounds to me liek you listen to rap (saw the boss hogg thing under your name) and for rap music JL might be a waste of money. sure rap will sound great on JL subs but youre paying for a lot of sound quality and you dont need so much of that for just rap.. a little will be enough.
im thinking some comp VRs, L5 solo barics, memphis PRs, or the ever popular 05 type Rs would treat you strong and youd be beatin a lot harder than the just the w3 alone.
but.. correct me if im wrong on what youre looking for in a substage.