Thanks Alex. I wonder what that other board is then. I have a theory on the enclosure. Maybe Rudy will chime in here. Its true there is an ideal enclosure volume for each driver and rarely is there room for an enclosure big enough as there is here but I am wondering if this was also designed to resonate at the lower frequencies.
Normally when you design the enclosure you don't want the panels to vibrate and color the sound but maybe in this case thats what they wanted to do. Two large panels made of thin material are likey to vibrate.
Alex, if you haven't put that engine back together again maybe you could touch the sides as you blow the horn or start up the prime mover? If you grip it tight and the volume decreases you will know maybe that was the intent of that enclosure design.