I'm having a hard time with WINisd. I'm trying to find a 15" speaker that I can match with a smaller cab that will fit in the back of my car. I thought I would use the Kappa pro 15LF, but it's only goes down to about 50hz, according to Winisd. If I plug in a delta, it wants me to build a cab that's 19 cubic feet to get down to 33Hz. What in the world am I missing?