Hmm. Something you should keep in mind is that the KEFs have a 3.2 ohm minimum, and if you're running the amps in bridged/mono mode they're seeing half that. I doubt the amps are rated for such abusively low loads, and it would certainly explain the heat and cut outs. You're probably triggering some sort of thermal protection circuitry in the amps. Hard to say about the driver. It could be the driver itself or a break in the circuit somewhere else (although the driver's voice coil is probably the weakest link).