So I ended up buying the GRS 15SSW-4HE from Parts Express. Installed it today in my 3.7cuft sealed box and went for a test drive. Didn't get a chance to check the deep bass response but so far it sounds great. Definitely more overall output than the 12 which is expected.
Today I noticed Parts Express sells a Single 18 with two 18 passive radiators as a DIY kit. I REALLY want to try this out, but I would need about 1000w @ 4 ohms. It would also require me to re-wire the entire vehicle since I would need another amp...I have a US Amps AX-2000 I could put on it, but that thing loves current.
Decisions, decisions.