I am following your plans for your tempest. I have a built in subsonic filter in my amp, but it is at 8 hz. So, I like your idea of the 16.5 hz tunning. My problem is that I cant find 6" pvc. Can I use 4" pvc, or 8" sonotube. And if so, what lengths would these need to be to keep the same tunning? Thanks for all replies.