Bass Box PRO is a sophisticated program. It does calculate the ports accurately. Having said that whether the port is 15.2" or 15.5" makes no audible difference
When you requested a change in the port opening dimensions, I ran the design again with your changes, and Box Pro gave a port length of 15.5". Now I re ran it, as Bass Box does take into consideration air resistance of the port, which is why the port volume changed slightly.
However, you have to look at the big picture, as whether f3 lands anywhere between 35 and 40 Hz there will be no audible difference to the human ear.
In closing I would say there is huge misunderstanding on the net, about what is bass and what is midrange. 20Hz to 400 Hz, is bass. Human speech is unintelligible with a sharp roll off at 400 Hz. The mid range is certainly 400 Hz to 3,500 Hz at least, and I put it up to 4000 Hz. That is referred to as the speech discrimination band.
That is the band where reproducing systems have to be incredibly accurate in their on and off axis response. The ear will detect even small irregularities in this band.
It is the accuracy in the speech discrimination band that determines more than anything else, whether a speaker is enjoyable long term. Of course you don't want huge problems elsewhere, especially nasty peaky tweeters. But I think you get the point.
Lastly no one will tell if the F3 of that speaker is anywhere between 30 and 40 Hz. It is of zero consequence.
I really don't understand, or even see any justification whatsoever for this "mid bass" sub concept. To me it seems totally daft and to have no sensible rationale.
The reason is that any sub that reaches 20 Hz, should have a driver with a good response to 200 Hz at least and I think actually 400 Hz, if it is properly designed.
One thing I would recommend you do, is to
invest in this woofer tester. They are easy to use, and it will give you the impedance curve of your speaker and then you will know exactly where your tuning frequency is, that you are so obsessed about. I would wager good money that the tunings of your current speakers are not where you think they are. But at least an impedance curve should be run on every speaker made DIY, that is a bare minimum check on the design.
