You also have to figure out if you want a sealed box or a ported box.
How much power you want.
For a quick idea if any of the cheap amps will live up to their "claimed" power ratings, you can do a bit of math for a spot check.
An alternator should put out a max power of 14.4v.
The amp fuse or fuses need to be totaled up, say that 1000w amp only has a pair of 15 amp fuses in it or 30 amps total.
You then multiply the 14.4v by the total amp of protection, in this case 30 amps to get the maximum those power those fuses will let pass.
14.4 * 30 = 432 watts available.
But you also have to factor in the amp efficiency, the newer amps do well around a guess at 80%.
Multiply your 432 * 0.80% and you can see only that 345 max watts will come out of that amp.
Don't trust less expensive ratings on amps.
Use this simple test to see if it will actually be able to pass the power they claim.
For example, I have this little Kicker ZX400.1 that I got for free as it was rain damaged.
I sent it off for repair and now it lives again.
It has a single 40 amp fuse and is rated to push 400 watts at 2 ohms.
14.4*40*.8=460 watts.
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It is currently hooked to a single vintage 12" Kicker Solo-baric sub in a sealed box in the boys VW bug.
He found these subs on Craigslist and the PO had them in these crappy home speaker boxes for $200.
One of the speakers has a tear in the surround, so we got him down to $125 for the pair.
We pulled them out of those cabinets and put the one into a proper sub box for the car.
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The combo does not miss a beat and will pound inside the car.