I just got my PA660 hooked up and running. However, on my ride home tonight, about a 45 minute ride, I had it rockin' out at about 3/4 volume, for perhaps 30 minutes and it went into thermal overload. It was HOT to the touch. It came back on after a few minutes, but then I couldn't even turn it up past 1/3 volume without it cutting out again.
I've got it mounted in the trunk vertically, with the power and speaker wires at the bottom and the controls at the top. It's off to the side of the trunk with nothing blocking it. I'm running it in 2 channel mode with the rear speakers being driven by the front channel, and the rear channel bridged into an 8" 4 ohm subwoofer, all Polk components. It's being driven by the rear channels of the stock head unit.
BTW, the line level inputs create noise, lots of it. I had to use a line output converter to clear that up.