I don't disagree out of hand with the common theories about power quality late at night (although my own measured line voltage doesn't vary much), but I feel (skewed, no doubt, by the fact that I live in the city) there is also a strong correlation between a relative lack of background environmental noise and times when systems seem to sound their best. As for mood, I find that the sound of my system changes my mood more than my mood changes the sound of my system - and if it weren't this way, I'd wonder what the whole point was in the first place! I do believe, however, that an absence of extraneous distractions while actually listening is very key, and this is often unlikely during busier parts of the day. (The topic of mood was often brought up in a thread I started a few days back [archived under "Best Of"] entitled "An unscientific poll: How often are you happy?")
Sound quality vs. Mood
I've been trying to get a handle on why my system sounds so much better some times than others. The most common explanation would probably implicate the quality of my power. That could very well be and as soon as I can afford to experiment, that's what I'm going to focus on. However, I've always suspected that mood has a huge effect - maybe the largest effect - on subjective sound quality. Any thoughts?