KEF is tauting a damping material called ACE (Acoustical Compliance Enhancement). It is supposed to increase the internal volume -- as seen by the woofer -- by a factor of 1.5 to 3.
There is a description of the ACE damping material here:
http://www.kef.com/technology/pdfs/acewp.pdf
Does anybody know if similar damping material is available from other sources?