Neither overdrive nor MSI Afterburner was able to clock the memory lower than 685. The settings just didn't want to take. I even tried using the unofficial overclocking mode with
Make sure the config looks like this for MSI Afterburner:
EnableUnofficialOverclocking = 1
UnofficialOverclockingEULA = I confirm that I am aware of unofficial overclocking limitations and fully understand that MSI will not provide me any support on it
UnofficialOverclockingMode = 1
AccessibilityCheckingPeriod = 0
And then when you hit the underclock limit, restart the application. It will then position itself at the previous clock as the middle, and you can downclock further.
I found that you don't need to actually restart the Afterburner.
Make your change on your first card.
Click settings and choose your second card.
Make your changes, settings again and choose your first card.
The change in cards is sufficient to allow you to drop mem clock lower. At least on my system anyways it is.
Saves a lot of time ;o)