Well do you want it to be loud or do you want it to be clean? You could buy some cheap ass amp and it will be loud. But it will sound like a peice of shit like those fucking black people cars with the trunk vibrating like a bunch of fucking springs are bouncing around. Spend the money to get a good amp. I'd go with diamond audio.
What are the specs of the speakers? Are the 2 ohm or 4 ohm? Whats the max power ratting and RMS? We can't just recommed AMP X without knowing what you got.
And also, your box and where your speaker fires also affetcs loudness. A sealed box will be cleanest, if you are pushing the right amount of power. Don't hook up a cheap ass amp to a sealed box and expect to be good. You need to push that shit hard. How do you want to face the speaker? Facing it to the back works great, but I prefer to have the speaker downfire on the trunk. I think its louder and hits harder, but thats my opinion.
Once you got all that figured out, then maybe someone can recommend an amp.