[MadMax]

Is there any diode protection to prevent the impedance spike from a disconnected motor from applying a high voltage to the driver board?

I like to protect anything driving DC through coils with a reverse biased diode in parallel to the component to clamp any inductance jump. I'm not sure if it's an issue with DC motors, but anything with a coil in it like a solenoid valve or even some mechanical relays can pop out a pretty high voltage spike when current is quickly interrupted.

You also get a lot of noisy commutator related noise on DC perm' motors. There should be a capacitor across the motor terminals. While the noise is mostly an issue with R/C cars (RF interference) you can get more brush wear and it can be a bit hard on transistors.

Is it possible to screw the heat sink tabs on the FETs into the heatsink? Thermal junction compound helps a lot, but it's conductivity is still crap compared to conduction thru aluminum. Minimizing the gap between the FET tab and the HS can make a big difference. However, if the AEG worked for awhile before it burned out, the thermal issues must be marginal. A goopy application of TJC should bring you well into the working range if the transistors died because of thermal issues.