POWER delivery... I had a home with problems on the Grid.. at times during the day between about midnight and 7 in the morning, (mostly when people were asleep) the system was in its best buttery and hi current delivery state, with no listener fatigue, however listen between about 4 pm and 10 pm at night and it was much more forward and thinned out sounding.. Obviously it has to do with the transformers and supplies to everybodys homes and all the appliances etc.. running during the peak times.. I could tell this just about everytime. Fixing it can become somewhat a messy solution to many and expensive.. Listen at midnight and then at 12 noon the next day on a saturday or something and see what you find out!!
Good Luck
Good Luck