the F15's are superb subs, but I don't think just one will cut it for you, unless you are in a small room. You could get one now and one later though, that would be a formidable setup. If I were you I would be looking at ported subs, hopefully two if you have the room. Two Hsu VTF2's or two Outlaw LFM-1 Pluses would kill for dubstep, and may be within your budget.
I have a few setups for music, my main music rig is some Hsu Research HC-1 speakers with a Hsu ULS-15 dual drive which is connected to a huge old Pioneer Elite receiver. This thing can pound out dubstep, D'n'B, or anything else, even the deepest pipe organ music with aplomb. The limit for my system is my ears, I can not take heavy bass all day, but once in awhile I will crank it to zero and feel the bass pulverizing my insides.
You don't need Klipsch speakers to have a hard-hitting setup, and you also don't necessarily need larger speakers if you are referring to floor-standing speakers. What you should be looking at is the bass extension spec rather than physical size of the speakers. Tower speakers are large because they have to be in order to produce a full range of sound, but since you are going to use subwoofers, the bass capability of towers is not something you will be taking advantage of; bookshelf speakers will do just as well, as long as they can reach down to 80 hertz. With that being said, there are some terrific and inexpensive tower speakers out there, take a look at the Infinity Primus p362's for starters. Also, a lot of positive things have been said about EMP's E5Ti's. Both of those are $500 or under. Ascend Acoustic's CMT-340 is somewhat large, although not a tower, and those are pretty highly regarded.
What also should be taken into consideration is your room and your speakers intended placement in it. If it is a small room, and your listening position is only a couple meters from the speakers, yeah the Klipsch would be overkill, and the Aperions would be nice in that role. But if you have some distance from the speakers, you will want some powerful ones that can project sound well.