The latest stable release would be the latest firmware posted in the latest SPS-UWX thread: Machinedrum SPS1-UW X.05 Released [ Unofficial ] .
Not sure about any considerations when downgrading your MC, make sure you get the corresponding manual? Also some functionality might break depending on how far you go back I guess?
All the firmware releases leave you with a fully functional/usable sans mc, yet still new and improved machinedrum.