Tuning Frequency for 2 15 Volfenhag Subs

I am in the process of building a vented sub enclosure for my brother's 2 Volfenhag 15".  I know they aren't the top of the line stuff, but they'll do.  What tuning frequeny would be good for the drivers.  He wants a SPL type system that is streetable. Here are all the T/S Parameters that I could find for the subs. 

Free-air resonance: 29 Hz

Total Speaker Q: .67

electrical Q: .806

Mechanical Q: 3.964

Vas: 4.6 cubic feet

Xmax: 16mm

SD: .0491M

This was all the T/S parameters I could find.  Also, 1 more question, does 1 2"x14" long port equal 2 2"x7" ports.

Thanks in advance for the help.

EBP of 36??  This is the kind of stuff you get with trying to work with Volfenhag specs.  If it's true...and I'm betting it's not quite...this should be nothing but a sealed enclosure.  (the EBP is Fs/Qes...under 50 should be sealed.)

It will be hit or miss working up a vented enclosure for this brand.  I don't know how you're managing to use these specs to make a box, but for what it's worth go for a tuning freq of 40 and use a subsonic filter at 35 Hz.

The opposite is true for your last question.  If you add another port the lengths of each one will be longer than the single port.

I suggest you talk your brother into investing a little bit more into his subs if he wants SPL...and wants his brother to do the design work.  It'd be easier on you, would be worth your time and you would at least have something to show for your efforts.

You might want to try using a single, larger diameter port tube that is adjustable, like this one.  Experimentation is the order of the day with the help of a test tone CD.  Be prepared to remove and replace one of the drivers for access to the port several times until you are satisfied with the box frequency.  Use electrical tape to hold the port through the trial process, and then when you are satisfied with the results you can mark the tube and glue it into place.