6th-order Bandpass

What lets a 6th-order bandpass subwoofer play so darn LOUD? I'm looking for SPLs and fairly tight punch. My current ported (EBS) box has slow bass, so I want something that will give me tight, firm, loud bass. I ocasionally take my stereo to parties, so SPL is important to me.

I want to build the loudest subwoofer I can with my two 8" pioneer woofers, and I've heard the 6th-order Acoustimass subwoofers and thought that they had some pretty okay sound for their tiny size.

What are the caveats that I need to be aware of, besides the uneven frequency response characteristic of bandpass subwoofers that might affect this 6th-order bandpass project?
 
What lets a 6th-order bandpass subwoofer play so darn LOUD?

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Gain and bandwidth are inversely proportional. If high gain is important, bandwidth must be sacrificed (and vice versa).
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I'm looking for SPLs and fairly tight punch...
....so I want something that will give me tight, firm, loud bass.

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Sealed and/or emphasis on frequencies around 50Hz would meet this requirement.
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My current ported (EBS) box has slow bass,

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There is no such thing as slow bass. Although a system with a fairly high group delay will manifest such a poor bass performance. Likewise when inadequate drivers are used.
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I want to build the loudest subwoofer I can with my two 8" pioneer woofers

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With those drivers, a 6th-order BP box is your best bet :)
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and I've heard the 6th-order Acoustimass subwoofers and thought that they had some pretty okay sound for their tiny size.

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some of those Acoustimass modules are of the 8th-Order type.
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What are the caveats that I need to be aware of, besides the uneven frequency response characteristic of bandpass subwoofers that might affect this 6th-order bandpass project?

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you'll almost surely have to build a big and heavy box :D
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Good luck!

:)