I looked at your pictures and my opinion is that your power amp has insufficient ventilation. After the sound deteriorates place your hand on the amp chassis if you cant press your hand against the chassis for 15 seconds or more, it's probably too hot (most people can tolerate 55 deg. C).

I'm not saying this is your problem, but too hot is bad news and certainly could be the problem. Also I'm unsure about the wisdom of putting the pre-amp on top of he power amp (excess heat again).