Hello. I just finished my first DIY build of the Fusion 8's for use as my mains on my PC 2.1 setup. For filling I used the following on pretty much every area inside except the very bottom where the ports are located. http://www.bondedlogic.com/acoustical-products/ultrasonic-pro-panels I used basically 3 - 12"x12"x1" sheets in each box and they are pretty dense. Does this amount seem like it may limit any output from the woofer? Thanks for the help.
I ended up pulling the speaker apart and doing some testing. The sound did open up when I removed most of the filling, but started to get a harsh at higher volumes. I ended up adding some back into the cabinet to about half my original amount.
I've had the same experience in general. If I over line the walls with something dense it seems to sound.. I'll say congested for a lack of better word. I'm glad you tested it because I'm about to build some Fusions too and had the same question. I typically line the sides with convoluted foam and put a handful of stuffing behind the drivers.