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:
[ATIADLHAL]
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.