No sense in arguing, the software has helped me do just that, many, many times. A smaller box can be tuned to the same lower frequency in this scenario, there are limits to this, sure, physically, but not within the differences we are talking about here. Optimal is the key, not bigger or smaller. If you can tune to the same frequency in a smaller box or lower Fb in the same size box, the entire band will look different, usually a benefit in a car environment when you employ either calculations using a SSF. When you apply a SSF prior to amplification, (an active filter as we are discussing here) BASS box pro still allows one to keep the tuning frequency the same or lower and allows you to use a smaller or same size box with varying results well within tolerances. Adding an external 12db/octave filter with a q=2 to a vented box whose resonance (fb) has been lowered can extend tend the bass response and produce a 36 db/octave cutoff, in some instances by as much as 12Hz - this applies to the same size box. If you use this in a smaller box and aim for the original cutoff, you can achieve this in the smaller box with the same frequency goal as the original non SSF filtered box, you can accomplish this in the smaller box. I have pretty extensive experience in designing home theater subs (in the hobby for 42+ years, subs for at least the last 35) where size is a major consideration for aesthetic reasons; the application to car audio is the same when it comes to the design of any sub for that matter. Granted, you may want a bump at certain frequency in a car sub that you don’t want in a home sub, either way BASS box pro will allow you to manipulate this when an active SSF is used in the overall design circuit and I'm pretty confident that the design engineers that wrote BASS Box Pro know a smidgen more about this than the two of us put together. 
