$600 for a couple subs is a decent budget, so you have the choice of buying some nice subs. however, nice subs tend to perform best with nice subamps.
so, if realistically, you will only have a budget amp that won't be replaced in the future, then maybe stick with some good but cheaper subs. otherwise, you'll be stuck with subs that don't reach their potential, and may in fact sound worse than a couple cheaper subs that are performing in their sweet spots. basically it's like the JL W3 vs W0 series, or alpine type-R vs type-S.
having said that, it tends to be the more expensive subs that perform better in smaller boxes too!
anyway, you should find the JL 10W3 and alpine SWR-1021D type-R subs to have a RRP around $300~349, so the streetprice for 2 of them will be within budget. these make a good reference point around which to compare other subs, so definitely worth a listen to them.
so what subamp are you planning on using. ideally 200~300WRMS per sub. and if much less, then consider the lower model subs that will perform very well with lower power.
the amp will also dictate what impedence subs you'll need (eg: 4ohm vs 8ohm).
good luck

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