First, there is an audio section you can post this at. Ontop of that the best for the price are the package deals you buy at best buy or someplace like that. In a tib a single 10' sounds good from what I hear. I have duel 10' Alpine R10's and a 1500w rock/fos amp, and I vibrates the cars next to me. Wanna change it out for a single 15' Kicker Solo B..
It's a hatch back so you dont need 2 subs. One presents enough bass for most people. The biggest issue is sound proofing the trunk. That is a MUST DO. If you don't you will vibrate your hatch to pieces. I picked up an nice aluminum cone Infinity reference with a MTX 300 Watt amp for under $100 bux on ebay and it's been working great! There are always a lot of good deals on MTX amps. They're good enough for just one sub.
personally me i would get it memphis . you can get a memhis 250 class d with a memphis pr 15'' sub and man youll be rocking the house use a ported box and youll feel the power. go to memphiscaraudio.com to look for a dealer near you this setup cost about 370.00 that includes the subwoofer,amp,sub box