there really is a misconception with sub sizes and their "range". An 8 has the ability to play just as low as a 12 or a 15. Now read that it has the ability, that doesn't mean it will do it as easily or efficiently. You've got to look at the specs of the subs you are deciding with, their thiele-small parameters, specifically the Fs (free air resonant frequency). It's a lot of technical data to understand at first, but taking those into consideration in combination with your box design (extremely important), as well as the quality of the amp and wire supplying it. Everything plays a role in how the subs will respond to frequency ranges in both terms of power and sound quality. If you have a smaller sub with a slightly higher Fs value that you'd like, you can compensate by tuning the box to a specific resonant frequency.
read up on this page about the thiele-small parameters:
Click Here
JL is great, but in my opinion they are overpriced, especially when you look at comparable products out on the market. and like he said, they are expensive, very expensive. the setup i like to recommend for rcsb is 4 10's. you don't need to worry as much about air space and can still incorporate amps into the box. And I don't like mixing sub sizes, it just isn't necessary. With that being said, I don't know your price range but my recommendations right off the bat are:
Fi X10
Team PSI Platform 1 10
B2 Audio IS10
DC Level 2 10
Sundown SA10
Skar VVX 10
All of those are in the same ballpark pricewise and are all quality manufacturers. look on places like ssastore and droppinhertzcaraudio, most anything sold on those places is some of the better stuff on the market. Then, you need a good amp to power those. Don't underpower them, if anything, buy an amp(s) that will push more power than they are rated but tune the amp down to save levels. Then you want good quality OFC power/ground cable to those amps, you could get away with 1 run of 1/0 and a dual 1/0 distribution block but I'd personally do two runs of 1/0 from the battery back. You'll want to pick up a better battery too if you are serious about it, especially if you already have accessories that draw more power. I run XS Power, so there's my preference. 1 or 2 D3400's under the hood is a perfect setup and either upgrade your alternator, put an overdrive pulley on your stock, or hope your non-modified stock alt can keep up. And do the big 3 upgrade if you haven't already.
