Did you hear a difference after them warming up? Maybe my ears aren't good enough, but with the J2 it sounds the same warm or cold.
I thought I did. At the time I was just following the instructions from the manual:
It takes at least an hour of warm up time to get the best performance out of the amplifier. It will take that long to reach operating temperature and exhibit lowest distortion and noise. This is not a subjective judgement, but based on actual distortion and noise measurements.
One of these days I'll check this myself, now that I have the equipment and the software to do so