Consumers are bombarded with all the information about subwoofers specs but what can be more important is to prioritize this information and of course pick what is right for you.  Obviously nothing beats a listening test but lets talk about what some people have considered iron rules. Lets start:

1. To be a good subwoofer xmax is paramount

Mostly true, keep in mind cone area matters. If wife acceptance factor is not a problem, a larger driver moving will displace plenty of air.  Resonant frequency, a T/S (Thiel - Small) parameter of the driver itself while very important is not going to be listed by the sub manufacturer unless it is a raw driver. At this listed frequency the impedance of the driver rises and it will resist the incoming power. Pretty hard to hit rock bottom when the driver is pushing back right? 

While it has been viewed larger drivers are slow sounding lets go on to ...

2. Large woofers sound slow, small ones are quick.

False, new motor technologies and lightweight but stiff cone materials let us use 12, 15 and 18 inch drivers where we used to think we needed a 10 for snappy bass. SVS has the new 16 inch drivers and let me tell you, there is nothing slow sounding about those...

3. Only a big box can emanate big bass

Well who can argue this one? Large box means we can have efficiency and deep bass. If we have a small box we can still have efficiency but not deep bass, or we can have deep bass and now we lose efficiency! Wow info overload...

This is what made Sunfire's compact subs remarkable, yes they are diminutive but they play deep but the horrendous driver efficiency, well not problem cause Bob Carver made a terrific high power amplifier to overcome it. MKSound has the new X12, tremendous output but the amp is >400 watts. No where near the Sunfire amp but both subs yield great output. I picked these two because what it comes down to is can you fit it? If I can I always go with the big box because it will need less LF boost to get the needed response. Hoffman's Iron law has done alot of the audio industry and while it cant be broken it has been cheated a few times. It is also the fundamental law that while observed by A.N. Thiel lead to the Theil Small parameters that are standardized as the small signal specs for raw drivers.

4. Ported subs are needed to go deep

Not true, while a ported box is most efficiency down low, the steeper roll off below the tuning frequency yields less bone rattling bass <30 cycles. Low end subs use small boxes with ports tuned high at frequencies we are sensitive to (e.g. 30-50 hz) to exaggerate bass. Anything below that and they are amp limited. At RDH AUDIO the audiophile grade subs we sell are designed right, if they are ported the enclosure size and tuning yields deep bass well into the low 20hz range and below.

Sealed boxes while they roll off faster have more control over cone movement and are harder to drive to xmax. This negates as much ULF (Ultra low frequency) high pass protection and also they roll of slower. This means bone rattling bass can happen with a smaller simpler enclosure. Downside is less efficiency but dont let that get you down! Just buy another sub!

If you have the room however and can fit one or even better two large ported subs, you are in for the holy grail! The low tuning frequency of properly designed ported subs also makes group delay less of an issue. Group delay was originally a audiophile 4 letter word ( not sure how to make that phrase a  4 letter word but it is, Okay?).  Fear not, modern ported boxes sound amazing, if you can fit them. Some of the most popular ones from respected manufactures are 25+ inches deep. If you have a false wall for your projector screen this is a non issue. Cylindrical subs from SVS also eliminate the large footprint issue.

5. Subs are best in multiples!

Yes! Absolutely, we recommend multiple subs and in my demo setup I use a stereo pair. I know Low frequencies are omni directional but the even pressurization of the room helps eliminate dead spots. There will be a upcoming sub placement guide to supplement the owners manuals of equipment purchased from us. Since most subs are powered, doubling the enclosures yields a 6db increase in output vs a single sub. This also means most likely 2 small subs will out drive a single larger one. Also in many instances it is easier to hide two or more small enclosures vs a single large one.

To sum it up....

This is only the tip of the iceberg, one thing that is super cool is the CEA-2010 standard for measuring sub output. Great reviewers out there have painstakingly measured the output of many of the subwoofers we like and with this standard they are held to a even playing field. Remember don't get too caught up in the specs, the best thing you can do is listen and with a expert sales person you wont be left astray. There are soo many great options today its not the extra db of output at 20 hz that will matter or the wattage of the internal amp but the sum of these components and how the sub will work in YOUR listening room. That's where we come in!