If you shorten the ports to move Fb (box tuning frequency) from ~28Hz to the drivers 35Hz Fs, output will increase considerably (though the F3 frequency will also rise) and the impedance peaks either side of Fb will be more matched.
By "nicely rubbery" sound, you may be used to an "underdamped" system, an Fb above Fs, which will cause a peak in the low frequency response around Fb, and "ring" longer than your "tight" overdamped response.
You trade SPL level for lower extension.
Most people won't notice a few Hz lower, but may notice a few dB less output.
Coming in at 34.3 Hz Fb is a good compromise
B&C's port area of 339.6 cm2 for the 18SW100 seems a bit on the small size for it's displacement.
What is the port area in your box?
Most likely the port's pipe resonance(s) due to it's length and diameter.
The pipe resonances phase go in and out with the driver's response and cause narrow band dips and peaks in the combined frequency response, something like this example:
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Nobody would hear the difference.
The tuning frequency would change more than that with output level and air temperature. Don't waste your time chasing the odd Hertz here and there!
As mentioned earlier, your impedance measurements indicate that the tuning is around 30 Hz. It could even be as low as 29 Hz. That's well below the manufacturer-recommended tuning of 33 Hz. As a result, you've probably lost about 1dB of response at 35 Hz. If you tuned the vent in this enclosure to 35 Hz, you'd probably get a flatter bass response in the 45–100 Hz region. This might add back a bit more punch to the bass response.
Of course, if you're applying a low-pass filter to your subwoofer, then the lower tuning may actually give you a little bit more low-frequency extension once the subwoofer's response is blended in with the main speakers. That will depend on the type of filtering that you apply and its cut-off frequency.
The above diagram seems to be a little misleading. I'm not convinced that the relationship mentioned therein is a given.
For example, using VituixCAD's enclosure simulator and the B&C Speakers 18SW100 woofer, I get the following impedance curve when the free-air resonance frequency of the driver (fs) matches the resonance frequency of the box (fb). Here the impedance minimum between the two peaks is located at 35 Hz (fb=fs). It is apparent that the two impedance peaks are not equal in height. This appears to indicate that the previous diagram is incorrect.
I checked the Thiele–Small parameters for the B&C 18SW100 in my edition of VituixCAD. They seem to be the same. It turns out that I had used an enclosure absorption loss of Qa = 50. This was enough to alter the heights of the impedance peaks so that they were no longer the same. When I changed to Qa = 100, the impedance peaks became equal in height.
