Your description of the nested sonso-tube sub is correct. I don't remember the details but the builder was very happy. I think that you could find details in the DIYAudio forum Loudspeakers subforum if you go back far enough. I think it was at least a year ago.

As for your PVC pipes. You are on uncharted ground as far as I am concerned so I am looking to hear about your impressions. Does a 3/4 wavelength pipe offer any advantages over a 1/4 wavelength pipe. I have no idea.

Martin