Car Stereos: subwoofers and amps, ohm subwoofer, 4 ohms

Question
I am trying to replace my son's subwoofers and amps that were stolen out of his truck recently.  I noticed online that there are 4ohms and 2ohms subwoofers.  Which is better and what does the ohms mean?  I will be replacing the two speakers with 2008 Kicker S15L7 subwoofers but don't know whether to buy 2 ohms or 4 ohms.
I also will be purchasing the amp for these subwoofers (that was stolen also).  What sized amp should be purchased for these subwoofers?  The watts peak/rms for the subwoofer is 2000/1000.
I am trying to suprise him with the new equipment so if I ask him to many questions he will suspect something and I don't know a thing about electronics.  Any help would be appreciated.

Answer
Hi Lori,

4 ohm and 2 ohm is just the impedance of the actual speaker coil in the subwoofer.  There is no musical/volume difference between a 1000 watt 4ohm subwoofer vs a 1000 watt 2 ohm subwoofer.  What it comes down to is how you intend to have them hooked up, whether in series or parallel.

If you're putting in TWO subwoofers, you have the option of getting 2 2 ohm subwoofers and putting them in series, which would result in 4 ohms of load across the circuit, or 2 4 ohm subwoofers and putting them in parallel and you'll get 2 ohms of load across the circuit.

Basically, most amplifiers can output more power at lower impedance levels, but you need to make sure that the amp you buy for the subwoofers can safely handle the speaker circuitry (or higher).

If the amplifier says it's stable to 2 ohms, you can put 2ohms worth of resistance across the circuit, or 4 ohms, or 8 ohms, and it'll be fine.  But you can't put LESS than 2ohms, or you'll fry the amp.

As for amplifier, there is no hardline rule for that.  Buy what you can reasonably afford.  Putting only say 300 watts through a 1000 watt sub certainly isn't going to harm it, but putting > 1000w will (you likely won't get an amp that reasonably achieves 1000w of actual power anyway.)

So if you're buying 2 subwoofers, here are the possibilities for configuration:

2 x 2ohm subwoofers in series: 4ohm (2 + 2)
2 x 2ohm subwoofers in parallel: 1ohm (1 / ( 1/2 ) + ( 1/2 )) (ohm's law)

2 x 4ohm subwoofers in series: 8ohm (4 + 4)
2 x 4ohm subwoofers in parallel: 2ohm (1 / ( 1/4 ) + ( 1/4 ))

So buy the amplifier first, and from there, you can decide the best option for the subwoofers.   If you get (for example) a 750watt mono block amp, stable to 2ohm, you'd probably do best to pick up a pair of 4 ohm and put them in parallel.

Good luck,
Justin