A couple days ago I was in the neighborhood and decided to stop by Mark'shouse to show him my RE MT 18's that I just put in my truck this last weekend.
When I was there I found out about his new SPL lineup of SSA subs, he had a
couple of 15" SPL prototypes in his garage that he was putting into a new box
which he was putting together that day. Here's a pic that I took, as you can see
it's a very solid design....
I believe he said the box he was making is going to be 8.5cuft tuned to 33hz with
130sq.in. of port. He is using the same amp as me which is a Rockford Fosgate
T30001bd.
Yesterday I came back to listen to the subs in the finished box and O M G...
This is the LOUDEST setup I've ever heard! It's even louder then the fully
loaded 18" BTL that I had which was getting powered by the same amp, but in a
7cuft box @ 33hz with 100sq.in. of port. It's also louder then both of my
MT18's, but hopefully that will change with a new box design

Right now I'm
tuned at 28hz. But regardless he was moving just as much air as me with the
prototype 15's.
It feels like it's hitting in the 150-153db range. I tried to hold up a newspaper to
the window of his car and it kept ripping out of my hands. The bass was very
violent and hard, hitting you right in the chest, it also caused some ear pain for
me too.