Jack, when it comes to subwoofers, you will find that there are a lot of different ideas of what is the "ideal" subwoofer. Keep in mind that with these, there is more to the equation than simply the speaker. It is really easy to make a cheap speaker in a properly designed box sound better than a high dollar speaker in a poorly designed box. So, keep that in mind.

Some other points to consider:
What size speaker are you looking to run? In short, the larger the speaker, the lower it can go. But, at the same time, the larger the speaker, the bigger the box that will be required to hold it and the less precise the music will be (the heavier center section causes the speaker to over extend from where the music directs it to go, leading to distortion). Keep in mind that most 8" subs require roughly 0.5 cubic feet of air space, 10" subs 0.75-1.0, 12" subs 1.50-2.0, and 15's you are looking around 4. This assumes a basic, sealed box. Get into vented boxes or the exotic boxes, thespace requirementsonly go up from there.

What type of music do you mainly listen to? If you listen to rock, country, classical, then sticking with 10" subs is fine for most applications. You get into some of the more bass intense music (dance, trance, rap), then going with a bigger speaker will benefit you.

How many subs are you looking to run? Most people run a single sub because under 400 Hz, the ears can not adequately distinquish between left and right. Since most subs are cut off at 150 Hz or less, having that stereo sound from subs is deemed pointless. But, with that being said, there are people that say they can hear a difference (I am one of them). The main problem with running dual subs is now the volume of the box has just doubled. That is now that much less space left in your trunk/boot. Some people care about being able to put a lot of stuff back there, others don't. How much room are you looking to give up? As a general rule, most cars with a 2 cubic foot box in them loose roughly half of their trunk/boot space.

How much power are you needing to balance out the system? The rule of thumb I use is the power going to your main speakers (the ones inside the passenger area) is the same amount of power going to the sub(s). So, if you are running say a 4x50 W RMS amp for hte mids/highs, then you need to run either a 1x200W RMS (single)/2x100 W RMS (dual). If you listen to rap/dance, then upping this power to 1.25-1.5 times is beneficial (ie, using the single sub power of 1x200, should be looking at something in the 1x250 to 1x300 W RMS range). Please keep in mind, I am stating RMS power, not peak power. Peak power don't mean squat really. Just makes it sound like you are getting more than you really are. Also, keep in mind that in a lot of cases, the more power you are running, the larger the box needs to be to dissapate the heat generated or else you can run into rapid subwoofer failures.

As for boxes, there are tons of options. Most people keep it simple and go with a sealed box. Simple, effective, but doesn't result in maximum sound. Hence why a lot of people want vented boxes. Gives more sound, but also doesn't feed back on the sub, leading to over extension problems with the sub. So, you end up with a "whub whub" sound to your bass (aka, sloppy bass). Me, personally, I like the single reflex boxes. This is a combination sealed box and vented box. The speaker sits in the vented box, but fires into a sealed box. The vented box lets all the sound out, but the sealed box keeps the bass as accurate as possible. The down side is the box size is twice the minimum required. So, lots of tradeoffs.

I'm sure some other people will come in with their opinions. I've had good luck with $50 MTX 10" subs, $200 Orion 10" subs, and also Boston Accustic 10" subs. Granted, the MTX and Orion subs were in a single reflex box and the BA's were in a sealed box.

Chris "Thermo" Coleman and K'Re Ann, the 03 X

 

Oh yeah, one other thing (like I didn't give you enough to consider already). The power rating of the subwoofer will also have a direct effect on the volume of the subwoofer. As a general rule, the higher the power rating the lower the efficiency of the speaker is (rated in dB/W). So, once you know how much power you are looking to run to the subs, you want to double that number. Having this number, this is the maximum power rating of the sub that you should be looking at. Keep in mind to keep it above the minimum needed to match the system. If running dual subs, then take these numbers and divide them by 2 (each speaker only carries half the load, but together they will carry the full load). This is what kills a lot of people because they figure if they get a subwoofer capable of handling tons of power that they will never blow it. WRONG. You normally can over power the sub (push it beyond its rated limits) and not cause any problem as long as the signal coming into the sub is distortion free. It is the distortion that adds to the heat generation in the coil, leading to failure. The distortion also can cause the cone of the sub to flex in strange ways, leading to ripping of the surround. So, even running the sub below its rated power can lead to an "overpower" failure. This is where clean amps come into play.

I design/build my own boxes. It doesn't take big power to get big sound. A properly designed system can give you tons of sound with minimal power. I was capable of flipping quarters on a Civic roof using two 10" subs and 75 W RMS going to each sub. But, I was using heavy gauge wiring, quality amp, and a single reflex box. All my normal tricks for maximizing volume output.

Chris "Thermo" Coleman and K'Re Ann, the 03 X

 

Well put Chris - this should be a sticky!

 

I have a Solo Baric L7 10" for the X. It's more than enough for this car. And this is coming from someone who did SPL competitions on a regular basis.

Ditto on going with a sealed enclosure. Vented enclosures provide more "boom" for your buck, even with crappy subs. However, SQ from these enclosures = teh poo.

 

What amp do you have pushing that sub?

 

Kicker ZR600. I like the old skool Kicker amps a lot better than the new mainstream stuff.

 

Extremely pleased with the Focal Access 25 A1 in mine.Recommended by multi award winning audio installer who did mine for the budget I had.