If the ready lights are on I would have expected it to run, 5v should be enough to drive it.
If the drive is active, the motors should either 'cage', hold position, or run away, if they have a tach wrongly phased.
You are most likely reading 100v to ground off the motor leads due to high impedance meter reading leakage.
If you are stuck, give HuFlungDung a PM over on the CNCzone site, he has quite a bit of experience with Bandit amps and repairing them.
N.

I sent him a message, he said he only had experience with older model bandits.  I also posted in the forum, but no reply.
I did a little more testing.  I found that if I put any positive voltage to H, with ground of that voltage to lo, or to the machine ground, OR any negative voltage to LO with the positive of that voltage to ground, then - The 100 volts going to the motor I measured goes to 0V. 
I am changing a part that maybe defect on the board, I'll let you know what happens after I swapped it.