I don't know about others, but I know that mine definitely won't run off +/-6V - it dies somewhere around +/-10V (and the bass gives out well above that). There is a little volume peak somewhere between those two points (although it's pretty distorted).
You don't need the SIM for testing. I have the parts for mine still sitting in their bags from Jaycar...
At least you're getting the same problem with both amps, so if you can sort out the power supply issue, you know that they'll both respond the same (as opposed to having one known broken one). Of course, if they're both broken, that's worse...
Are you using the safety resistors? With them installed (and the full power supply), it's pretty hard to go wrong - measure the voltage drop, like Rod describes, and make sure it's within the limits - if it is, then it should be pretty safe to leave it on and poke around, even if it's not working properly. I think I calculated the current drawn by my amp (per rail, one channel, during initial testing, no bias) to be about 90mA.
I don't know if it was just me, but the first time I hooked mine up (no load), I measured something like 25V DC offset on the output. The moment I connected a load it disappeared, though. Both channels did the same thing, so I guess it's right...