There is no option to disable sub-woofer in XPS system.
You can change equalizer settings in audio application in the system tray.
Below are the steps on how to change audio settings.
1: On Windows 7 systems, click Start --> Dell Audio--> Dell Audio on Windows 8 systems, type Dell Audio from the Start Screen, and select Dell Audio utility.
Dell Audio by Realtek utility is launched.
2: You may change the desired settings from the different tabs available on the utility.
Thanks and Regards,