Just today I had a JL Audio Stealthbox with a XD-1 600 Watt amp installed. Makes a big different in adding moving more air with the bass. Nice thing about the Stealthbox is that it looks completely stock.
Don't know how much more you would need, unless you are planning on also installing 24" DUB's and scissor doors too. I had a pair of 12" built into a custom enclosure with a 1200W amp driving them in my Roush Mustang and they didn't provide that much better sound - for at least 5x the cost.
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