I installed Version 6.640 on my 898 C SI unit and it's working good. So far so good. I installed it on 7/23/13 and have used the unit about three times since then. Each time I played with all the setting for a good 6 hours each time out. So far every thing seems to work ok.
I've been playing with the Noise Filter settings, Surface Clutter Settings, Sensitivity Settings 2D, Frequency settings and the fish sensitivity settings among other settings.