Which sub should I get?

Larger subs are better at producing the lows and SPL. Go with a single 15.

DC Audio
Fi Audio
Sundown Audio
Ascendant audio

I recommend you 12' Hertz.

X10s have the same xmax as x12s and should provide comparable performance in the low freqs, of course that assumes an equally high quality enclosure. With the reduced cone area they won't be quite as loud.

Ik

I run a single 12" Alpine R-type and have been nothing but happy with it. When you say "I've heard Rockford, Kicker, Alpine ect", what exactly does this mean? It's one thing to "hear" a speaker that some Johnny Amateur put in his home made un-tuned enclosure and hooked to his el-cheapo Boss amp. It's another thing entirely to hear a speaker put in a properly tuned enclosure with a high quality amp. I know what my single 12" Alpine hits like, and I've been told by professional installers that it sounded to them like I was running TWO 12's. So...keep that in mind when you're listening to your buddy's systems.

While I like Sundown, I think they make pretty decent equipment, I've never personally used their subs. So I can't rave about or against them. What I have used is Alpine, Rockford, Orion, JBL, and even been through an old set of Pyramid Super Blues (back when they were still made in the USA...still got those and they still work great). I have to say the best "bang for the buck" is the Alpine R's. You can pick up a 12" R-type on amazon for $150 or so. I feed mine a steady diet of around 750 w RMS (max RMS is 1000) and it runs all day long, low and strong, and push it pretty hard.