The hollow concrete tubes go by the trade name Sonotube. I've used 'em, but to build a deck and fence (I know, I've failed as a 'phile so far). The exterior of the tubes are shiny and if I had to guess, reflect more than absorb but I really have no practical audio experience with these. In any event you may want to check out HSU's subwoofers, I suspect they use Sonotubes in their design.
Tsouth, your company firewall blocking you is somehow amusing to me.
Tsouth, your company firewall blocking you is somehow amusing to me.