The STUD Woofer is for people who want an in-wall subwoofer that is a near equal to the sound quality of the best cabinet style subwoofers without the inconvenience of fitting another 'box' into their living space. The crossovers for the SE10SW and SE10SWD are sold separately because you may not need them.
If you use a single in-wall subwoofer, use the dual-voice-coil version (SE10SWD). It connects to both the left and right signals from your amplifier. If you plan to use two subwoofers (left and right), use two of the SE10SW versions.
If your stereo has separate low-pass speaker-level outputs for the subwoofer, you will not need an additional crossover. If not, you will need two crossovers (MX10's); One for each channel.