However, they are not inexpensive. Next choice would be good quality weatherstripping to make the door seal as close to airtight as you can.
I believe in part it is due to a poor seal, even with weatherstrips around, it doesn't seem to be doing a great job, this morning I had heavy rain on the roof was very noisy and was muffled within the room but still quite audible, I then held up some left over insulation and it made a huge difference, could hardly hear it, so I am thinking could I add another door and have some insulation between the two or would that create a different issue?
Got an update from Mark recently, upgraded the submersive to a HPi+ amp. Is sounding great, tight bass with great authority. Looking forward to a matching slave coming in the Aussie GB. Hope to have more woofage by Xmas