Thanks for the suggestions. The amp is based on a similar chip as the T-amp, which can handle 4 or 8 ohm loads without any problem. In fact, I've been using my little T-amp in the meanwhile with my m80's, and it can handle the load without any problem. I hope this one doesn't blow out too, but it isn't running too hot. Since the technology is the same on both chips (switching class-D amps, though they like to call them class-T), I'm not sure why this would be a problem on my multichannel board, and I'm pretty sure it's not that it can't handle a 4 ohm load. What I have NO idea about is whether or not having two different sets of resistance (4 + 8 ohm) could be contributing.
