i like your set up so far. honestly, it is a real system. but when you are looking at subs, remember it is a "buyer beware" market. just like loudspeakers are subjective, so are subs. i like LOTS of bass, i like it to get everywhere, and i like it tight and musical, so im kinda into the 18's sealed. but if you are thinking about running a smaller sub, think proximity. get it closer to where you are listening.
if i was hamstrung to a budget, i would save a bit and double up on the BIC (for my own tastes). it digs deeper-ish and the price is great.
if i had the option, i would save up and enter the 2-3000 dollar range and get something with an 18..... if it comes with DSP you can always tune it.