Hi as the MK808B has been changed with new Wifi chipset. The new ones use the Realtek RTL8723 wifi module, so you also need the new firmware.
Download link: MK808B New Firmware
How to tell if you get the new bulk or old bulk?
It’s very easy, the new bulk with RTL8723 chip will have the series number in the mk808b, such like the following photo:
Update on 17th July, some customer said the stock firmware have video stutter problem when you use XBMC, so here is a custom rom which fix this problem