re se series ported

WinISD is going to look for the flattest FR, so that is why it will default to the lower tuning frequencies (in many cases).  You may want to experiment by doing this:  use the program to model several responses for the same sub.  Do this by choosing New Project > select your sub from the dialogue box list > in the box Choose Alignment, select a different one than the first one that defaulted...

This is excerpted from the WinISD sticky subject:

Fiddling the response

Now that you're becoming comfortable with making changes and seeing that nothing has blown up, you can start fiddling with the response from your woofer.  Change the airspace to several different volumes, watch the chart for the response, and note the Q.  Bring your same woofer up again on the same chart by clicking 'New project', select your driver (it's now on the list), and click through the "New project...' dialogue box making whatever changes your whims allow.  Change the color of the response by placing the cursor atop the colored bar in the box and click to select another color for the second response line.  You can compare any number of different volumes/ alignments on the same chart by using this method.  multiwinisd.jpg

Compare the results.  You can also change tuning frequency on any open model by highlighting the frequency window and typing in the Fb of your choice.  Look at the displayed result.

Always check excursion and rear port air velocity charts when modeling a vented box.  Input the power that will be given this sub at RMS under the signal tab to get a graphic response.  The excursion will compare to the listed Xmax of the sub; you don't want to model the box so that this excursion is exceeded.  The air velocity chart will show port noise, and you want to keep it below 110 ft/sec.

Keep plugging away with the program.  The more you try different choices and note the changes in responses for each change you make, the more you will become aware of how the differences in box volume affect the subwoofer. 

Personally, I tend to try to tune lower than the common 40 Hz that is so often seen, for best SQ.  But the tradeoff is larger overall box size and special attention to those critical factors I mentioned above.  You have to make the sub fit the car, and you may want greater SPL than I would look for...so tuning higher than the program's "recommendation" would be appropriate for you.

EDITED TO ADD:

I looked at your subs, first by going to RE's website to check recommendations.  They say to use 1.25 cu ft per sub tuned at 34 Hz.  This will give you a "typical" car audio SPL response with a boost at around 40 Hz of 3.5 db.  For two subs, of course, that is a 2.5 cu ft box (net air space, not counting what the port takes up).

And, by the way, inputting parameters gave errors.  I traced it to the Bl listing of 17.7.  Using WinISD's auto-calculate, the Bl should be 12.65530 with all other parameters as listed.

You can lose a bit of that 40 Hz boost by lowering the tuning frequency just a few Hz, which will start flattening out the response curve.  But if SPL is your goal, just stick with the manufacturer's recommendation.  Also note the other graphs:  you will need the equivalent of a 5" round port diameter to keep port noise in check, assuming a total 1000 watts.  The port length would range in length (with a 5" opening) from 14" to 20", depending on where you want Fb.  The longer length lowers the tuning freq, flattens the response a little, and will sound better, but loses 1 or 2 db's.

Also note the excursion:  make sure you set the highpass filter on your amp at 20 Hz with this vented box.  This will protect your sub from over-excursion at high db levels with very low sub tones in the source material.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.