Help with understanding Transfer Function Magnitude

[FantasticBass]

So I attached a PDF of the TFM from WinISD Pro for the box I'm designing for my two Sundown Audio SA-15D4's. 

So I know that a lot of people will use this program to see how the box will "sound". So by looking at this graph, how do I know how it will sound? 

And in explaining the theory of it to me, how will this design actually do with my two SA-15's? What I can understand from the graph is that it will be a low end monster. Again no clue what I'm truly looking at so, just guessing really off of what information I've found through researching on the internet via google. 

I appreciate the help!

transferfunctionmagnitude for two sa-15 @ 1500w.pdf