Could also be because that particular driver has a natural drop off around that frequency.fusseli said:That would have been either open-ended with no LPF or an LPF very high, e.g. 250Hz. I don't remember which exactly, it was a few years ago.
I wouldn't run a down firing sub as high. By all means, share some more open ended sweeps of down firing projects. WinISD is an approximation in its best form, it will tell you nothing about a lot of important factors. Any sub may integrate best at different xover points for different mains in different environments. That's a no brainer.
What i posted is an 8" Dayton RS sub, otherwise good a ways higher. I thought it better to post a measurement of a down firing sub and leave it for interpretation. This isn't an easily answered question.