You might want to try something like hornresp with more complex induction modeling. It will still be similar, but will show close to reality than winisd.
WinISD is a quick tool to use, I took some time to throw it around in a few boxes tuner low and high (30hz) on hornresp and yea it was slightly different but the overall was still there. I’m curious if these have the same response in a 6th order box as the DS115.
If I had to guess you see more 8 ohm subs (and SVC at that) because they anticipate them being run in multiples, or if for pro audio, in horns or open baffle, both of which require significantly less power.
That is correct Sir...... SMALL gigs have 8 subs.....
The filter I believe you speak of for a tweeter is to prevent a peak in the shape of the passive highpass due to the impedance spike at the tweeter's FS.
Since subwoofer systems are normally active there is no such need for a filter like that at the FS of the driver since they are unaffected by impedance. That said you still need to employ a high pass if using the driver in a ported enclosure in order to prevent over-excursion below the tuning point of the enclosure where backpressure no longer keeps the cone excursion in check.