So I'm curious... I've never hear anyone elses IB but mine and I wonder if the results are the same. I have 8 15" woofers driven by 125 watts each (guess that's 1kw). It is incredible. I makes my vision blurry because it shakes my body faster than my eyes can track... like you're in a giant paint can shaker. My inner ear gets rattled and I get disoriented. It drives the air in and out of my lungs. It feels like a physical assault. It can be very alarming. Guests were frightened during the earthquake in Ocean's 13. They had no idea an audio system could do that. Is this what IB people are ending up with?
Edit: I think 2 15 inchers have equivalent cone area to a single 18 incher, so I should have the same air movement as you. I had to fit them between 16" studs. Also, mine's not a manifold like yours, it's a linear array along the floor.