The Yongnuo 560 II for about $70 (Amazon) is as powerful as the SB 900 and can be triggered by an optical pulse (i.e. SU-4 mode) even if you are triggering others using CLS. Put the 560 in S2 mode, manual power, and it delays the flash to account for the pre-flash. It may also lower the camera's sync speed a bit; not sure on that. My suggestion is that you consider the Yongnuo as the second or third flash.