[Mike P.]

Fluttering is usually associated with air leaks. Did you use weather stripping between the driver frame and the cabinet? Modeling shows you should also be using a Hi-Pass filter at 20 hz.

 

[Mike P.]

The box joints are a priority as well. If you're not absolutely sure of their integrity, use a latek caulking compound on all interior seams. If you use silicone to seal the seams then let it cure first before you mount the sub. Silicone fumes have been known to eat the glue that holds the surround to the frame.

maybe a passive 6 or 12 db on the speaker line?

As long as it's effective around 20 hz or so.

 

[Mike P.]

Modeling shows the subs limits with your setup, a Hi-Pass filter at 20 hz would eliminate any problem you are experiencing.

 

[Mike P.]

Whatever you decide to get just make sure the output you're looking for is within the subs capabilities.