I have a FV15HP. It was the first "real" sub I got, and it replaced a little KEF 10". It is indeed a beast! For the first time, I could make my glass of tea skitter across the table if I wanted. Set it up according to instructions, (I thought). While playing with big boom movies and high volume, I heard some distinct clicking noise at the big booms. I thought it was my towers. It didn't even occur to me that this 15" beast could be the cause. I called Aperion and played w/ crossovers and various settings. Finally noticed that even w/ very high crossovers to my towers, I still got the click. Figured out it was the sub. Called Rythmik. He told me to turn on the limiter switch. I did. Problem went away. Put tower crossovers back where they should be. Life is good.
I'm not smart enough to understand the design reasons for a limiter switch, and under what conditions you would turn the switch off. But I'm not smart enough to understand a lot of things, and choices are good. Now properly set up, this sub is good and strong. It is now one of two subs in my fairly large, high ceiling room, and life is good. I do not anticipate wanting to replace it before it dies.