If you wanna get stupid loud and don't really have a budget you could probably fit at least 4 12s back there. Scrap the spare tire, drop one in there, then build a box to hold the other 3 one centered between the back seats and the other two angled against the left and right corners there. Bolt your amps above the speaker box to the back seat.
Bam, stupid loud.
Personally I don't like subs in the trunk, they take up too much space and I use my hatch for things. I have an amp back there but it just runs all of my interior speakers. I have thought about hiding a sub under a seat but don't get to listen to music as much anymore so what I have works well. Coolest set up I have seen was an 89 MR2 that had a giant amp behind the driver and a 12 stuck behind the passenger seat. No Idea how they fit it back there.