Using the same computer with Windows 8 and Ubuntu I noticed that the sound was worse in Ubuntu and I discovered why soon: subwoofer doesn't work out of the box!
How to fix it
The fix is quite easy to apply (but it was not easy to find the right one!). First of all edit /etc/pulse/default.pa and add this line at the end:
load-module module-combine channels=6 channel_map=front-left,front-right,rear-left,rear-right,front-center,lfe
then edit /etc/pulse/daemon.conf, modify the line "enable-lfe-remixing: no" to "enable-lfe-remixing: yes", then uncomment it (remove the semicolon in front of it). Reboot your PC and enjoy the subwoofer!