Lonely Raven,
First, quite impressive you can identify a resonance at 15-20Hz.
It's unlikely that any rigid fiberglass or minerwool will affect this range.
Best bet is to damp the material...in this case (and it would be trial and error...don't think you'll find anything in a book to assist):
- glue and otherwise properly secure a layer of a 2#/ sq ft, 1/4" Limp Mass Barrier (note, not the more common 1#, 1/8")
Look at:
Soundproofing Material, Noise Control Curtains, Acoustical Panels, Soundproof Walls, Foam
that should damp the material to the point any "ring" is less or non-problematic.
WOW...I'm guessing your sub is setting this off???