I'll throw Image Dynamics Q series into the mix (IDQ's). Their new power rating I believe for the 12's is 350W RMS max, so 2 of these will mate to a 600W amp nicely. They are a very SQ oriented sub, but do not let that fool you - they are loud! A small sealed box in the neighborhood of 1.15 cu. ft. will work well with them. I have the older series (the new ones are known as V2's) and have had no problems with them other than a bit of hearing loss

I believe their MSRP is $200.
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1991 LX Hatch 5.0L