Do I need the X-Video Big Ben option to lock to Video?

No, you do not.

The standard Big Ben can lock to analog video, NTSC or PAL, and all the usual sample rates and pull up/downs. The X-Video option gives the following extra functionality. 

• Ability to act as a video clock generator.
• Ability to act as video slave (like a standard Big Ben) but whereby the video outputs are genlocked (cleaned up/regenerated) versions. The X-Video card will maintain the color-phase and field information through “Genlocking”, while simultaneously stripping the original signal from its pixel information.