I built this box yesterday. It is 1 cubic ft which is what JBL suggested. I have a few theories. First, my amp doesn't have enough power. It can push 300w mono. The speaker can handle 300 rms and 1200w peak. I have read that the amp needs to be able to push between 75-150% of the speakers capability. Second, the sub is not sealed completely. Every corner, edge, and crack around the box has been sealed with silicone. I can feel air escaping between the box and the screws that are holding it. I added carpet and a thin piece of duct insulation/foam, and now air feels like it is coming from just inside the rubber that surrounds where the screw holes are for the sub. I have had these subs for 7 years, but haven't used them in about three years. I have tried both of them and they do the same thing. My final theory is that the magnet is contacting the box, inside the enclosure. I am pretty sure this isn't happening, but I am running out of ideas. Here is a vid I took about an hr ago. It sounds pretty close to this in person. You are able to hear much more bass though.

https://www.youtube.com/watch?v=nRLe...ature=youtu.be

 

What does the sub sound like out of the box?

 

Sounds like the dome is separating from the woofer. I had some subs that one did that on and it sounded just like that. Almost a blown vibrating sound

 

Ill take it out and listen to it out of the box tomorrow.

 

Well I'd like to say I am smart and figured it out, but really I just followed your directions and took it out of the box to see what it sounded like. It sounded the same, but as it was sitting out of the box it started vibrating more than usual and was making the same noise louder and louder until finally the JBL logo piece just popped off. As soon as it did I thought, great I'll have to throw it away, but it sounded like it was brand new again. So, after all of the trouble I have gone through trying to seal it up better, it was label vibrating. It sounds and works great now. Thanks for the advice.

https://pbs.twimg.com/media/BjiYNdTCEAAQVQQ.jpg

 

Glad you got it figured out. My dome didnt pop completely off, so i just glued it back with woofer glue and let it set up for a day before i played it again.