I apologize in advance if this seems like dumb advice. Living in central NJ (as your tag says) you will assuredly be able to find a quality subwoofer in the used market. Subs seem to hold up pretty well through one or two previous owners. As long as it's not too old, it should be plug and play, so to speak.
My rationale: An $800 sub (used for $500) will usually outperform a $500 new sub.
That said...If I had the money, I would never buy a single used item again.
I previously owned a $499 subwoofer and in comparison to a $1,000 subwoofer, it seemed like a $50 sub. Just remember that price is only the amount you pay, and doesn't necessarily dictate results.
Just for reference, is he looking to maximize SPL, or just even out the sound and add some dynamic?
Where did all my money go?