A pair of these…
Dayton Audio SUB-1200 12" 120 Watt Powered SubwooferDayton Audio SUB-1200 12" 120 Watt Powered SubwooferAdd thrilling low-frequency effects to your home cinema experienceThe Dayton Audio SUB-1200 12" powered subwoofer system is the perfect addition to any home theater system. This powered sub...
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RSL speedwoofer is 430ish. More linear, but having duals for Better FR potential, and a little more headroom goes towards the Daytons.
Also as
@vader540is said, the used market is great for budget shopping, and the NSD is a great choice if you can find one in your area.
That of course is relative. But the 400 dollar subwoofer range is tough, believe it or not.