Hi robg1976, Using a filler material in your speakers will, as Fraziel has pointed out, help tighten the bass and you can also expect better soundstaging and imaging too. I have tried a variety of filling materials including dry sand but have always found Atacama Atabites (for more information visit: http://www.atacama-audio.co.uk/atabites.htm) to offer the best performance although it is the more expensive option. Whatever you choose to fill your speakers with it is worth filling them in stages: results are obviously dependent upon the speaker but also your room and your listening preferences. Hope this helps! Kind regards, Darren