I recently bought 2 of the Ultimax 12's and put them in a ported box that was an exact replica of an SVS PB+2 that I previously owned. I was getting that same type of sound that swc11 was getting at about half excursion, even when I sealed the ports??? No one could tell me what it was. I went to the sealed boxes at PE and they sound great now.
Problem is on the Ultimax subs one person says they were designed for a sealed box and the next expert says they perform better in a ported box.
So now I am so happy with the dual 12's that I am thinking about building dual 15's with the PE sealed boxes;) I also sealed the ports on my SVS, so these are my first experience with sealed subs. I have always been use to the ports pressurizing the room and digging a bit deeper though.