With a DIY build the Dayton Titanic MkIII is a formidable competitor. They do quite well in sealed enclosures and get pretty low for a sealed system with very little eq. (The Dayton amplifier has a parametric EQ.)
My ultra was no comparison, but I have not heard the PB-12 Plus. Then again I have not heard the Titanic in a low tuned vented application either.
