What size subwoofer to get?

Stehno hit it on the head.

Rives also brings up some important points. One sub IS harder to place than two. On the other hand, two well designed commercially built subs can be quite expensive.

Just keep in mind that a vent of any type ( ported, passive radiator, slot loading and yes, even transmission line ) will have slower transient response ( reduced impact, less definition, increased ringing, poorer decay, etc... ) than a well designed sealed box would offer. The sealed box will also TYPICALLY have less problems with interfering in the upper bass since there is no leakage through the port or passive radiator "talking". Placement should also be easier since you only have to worry about room reinforcement / nodes from one radiating surface ( unless you have multiple drivers ).

The major drawback to sealed woofers is that a good one will be quite large. Having said that, two good quality sealed subs that are offer large box volumes would be VERY hard to beat if properly set up.

As to the Bag End, it is equalized to produce 8 Hz, but only at very low volume levels. As driver excursion and spl is increased, the bass processor rolls off the extreme lows so as not to muddy the more prominent notes slightly above this range. Kind of like a smart "loudness" control that one can find on receivers, etc... The biggest problem with Bag Ends is the construction of the cabinet, which is not as rigid or damped as it should be. It also takes a pretty good amount of power to really make this thing sing, which is true of almost all subwoofers. Sean
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To add to Rives' comments, front loading and downfiring will have DRASTIC effects on bass output and pitch, even if crossed over at or below 65 Hz. I would suggest listening to the two different designs side by side to see which you like best. My experience is that front firing will typically sound more musical with better pitch definition whereas downloading will provide greater "oomph" with less attack and tonality.

One should also take into account that downloading places more stress on the driver itself ( the suspension can sag pretty drastically over time ) and a system of this nature will require a slightly sturdier amp with greater current capacity / higher damping factor. Sean
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Danner's comments about surface area only cover part of the equation. Not only is surface area important, so is the amount of displacement. In order to achieve high levels of displacement, a "long throw driver" or one that is capable of making quite a bit of excursion is required. This is especially true if one wants to or is stuck using a smaller driver. As such, one can find an 8" driver that actually displace more air than a 12" driver. It can do this even though it actually has a MUCH smaller surface area due to the longer stroke that it can take. The downsides to such an approach are that the driver will be harder to control due to increased amounts of reflected power and may generate higher levels of distortion.

Like anything in life, there are trade-offs involved. You have to weigh the variables and pick the lesser of two evils. It is rare to find any given product to be a "clear cut" winner in every category. Sean
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