I just finished building a sealed cube subwoofer. Here are basic bits of info on it: [*]Internal volume of enclosure: 3.9 cuft (20"x20"x20" external)[*]Internal volume of enclosure after subtracting amp, bracing and driver: 2.8 cuft[*]Driver: Shiva Mk 1[*]Amp: RCM Akustik Detonator 110 Mk II (>150W RMS, No bass boost, etc. 182 KB PDF: here)[/list]I have heavily braced this box, and have done everything I can to ensure it is sealed. The box seems very tight, and pushing the driver in reveals no leaks and seems to feel quite different than when the top was open. I have also elevated the box on spikes. The problem is that the bass feels boomier than my previous 6 cuft ported sonosub, when I would have expected a smaller, sealed low-Q sub to be tighter if anything. Another difference with this sub is that I have wired the dual voice coils (correctly, as specified in the Adire spec sheet) rather than gone with a single voice coil in the sonosub. The boominess seems to be exacerbated by some specific types of music. The worst occurence occurs with a song that has a very periodic deep bass. The bass will literally travel in and out of sync with the mains sometimes being delayed by up to half a second! I have used approximately 2.5 lbs of polyfill - I experimented with other amounts and that does not seem to be contributing much. I also experimented with phase and reduced the problem somewhat. Any suggestions?