The best value for the money I know of is hsu research. You can get a used vtf hsu sub on audio groan for about $350-$400. WELL worth it. Dr. Poh Tser Hsu knows what he is doing with regard to subs.

I don't know of any subs for less than that, unless you get a used velodyne. Can't think of anything else in the sub $350 range that would work well.

The dca drivers in a PAWO horn deserve a good sub.

I'd save. Besides, a good sub can be used in other systems as well.

My 2 cents.