If you're melting power supplies in a DIY amp my first thought would be ultrasonic oscillations (no I'm not kidding). It's pretty easy to fix once you know what the problem is -- even a simple RC filter on the amp input will cut down the frequencies above audio enough.
There's a possibility that driving a lower impedence load might make the amp more prone to oscillate (although I don't remember why) but I doubt that mixed impedences have anything to do with it.
The symptoms of oscillation would be that one or more amp modules get VERY hot and either trip the protection circuitry or "let the smoke out".
It could just be something boring like a short, of course
