Texan Sonic dampers for speaker cabinets and subwoofers are, alas, no longer available. The problem with trying to use materials to damp the cabinets resonance is that almost all if not all materials trap as much as absorb. That's why Tekna Sonic dampers were so effective, they were a mechanical diode. They were also excellent for use on room walls as it tuns out. I used to use Tekna Sonics for my isolation stands way back when was big customer. IIRC I bought the last Tekna Sonics available from Music Direct eons ago.
