You should never need to reflash the box, unless you don't know which version of firmware was on there in the first place. Even then, it's been suggested that the system files on the DMS probably won't interfere with things so long as they are for the appropriate hardware.
Also, as long as you've used the DMS tools to deal with the DMS', and not tried to do anything else (like delete partitions and re-create them manually) everything should be in good working order.
Also, it's worth noting swapping cartridges/drives in out of a Keg can often get the head unit confused! I have to say I've never tried swapping complete boxes - so who knows where it's at now!
That aside, I'd recommend doing the following with the 710 and the 10GB cartridge:
Remove the files 'forceupdate' and 'forcesettings' from PHTSYS.
Make sure you have at least one playlist on the DMS.
Plug the cartridge in, and see what happens (paying attention to the lights if nothing appears to be going on with the Head Unit)
If the head unit doesn't play ball:
Disconnect the Keg from the head unit (at the Keg end should be fine)
Perform a factory reset on the head unit
Re-connect the Keg
(There's normally a little hole for the reset button which you need to keep depressed for so many seconds. Refer to the user manual for your headunit for guidance on this)
If after that, there's still nothing going on - we'll have to dig a bit deeper.