Sonnie is right on, two more speakers in your manifold and that thing will rock!
The two most common comments from guys who have an IB with two speakers especially 15" is #1 small room with door," sounds good" #2 room open to the rest of the house " my box sub kills this thing!" neither comment is exceptible. IB's are known for being very musical and having enough SPL @ super low freq. to knock your house down
Your problem is not with your equip. and REW testing won't fix it, IB's work completely different than boom boxs, there is no box to make the boom therefor, there is absolutly no boom just clean sound the way the sound track was designed to be heard. Again add two more speakers and amaze yourself and friends.
Hope this helps, Rick.