With $400 there's plenty of good amp choices. I like the JL and the PG Xenon, because they are so flexible. You can change subs and not have to worry about switching amps because they put out the same power at any impedance. The Alpine PDX is tiny, a good choice if it's going under a seat or if you want to cut down on weight.
JL 500/1
PG Xenon x600.1
Alpine MRD-605
Alpine PDX-1.600
Diamond D3 800.1
Memphis MCD500.1
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