Different & inconsistent sound quality


I am noticing different sound quality from my audio system, depending on when I listen to them. My system consists of Krell integrated amp, B&W CDM 1SE, and Sony DVP-7700. They are connected with Transparent ML Super interconnect and MIT T2 Biwire speaker cable. For example, in the moring and very late night, piano recordings sound very clean, detailed, and transparent, however, in late afternoon or evening, it seems that some of piano notes from the same CD sound very dull and veiled. The phenomenon is true of other recordings of cello, violin, and etc. I suspect that AC noise has to do with the problem, so as a first step, I consider upgrading stock power cord for my integrated amp to a better one such as Transparent PowerLink Super or others. Could anyone help me with any possible solutions to the problem I'm experiencing? Any suggestions or comments would be greatly appreciated.
jwminf9e7
Thanks so much for responses. They were quite informative and helpful. I guess that power noise problem seems to be a more seious matter than I initially thought. I will look into all the suggestions and report back what happens. Thank you so much again!!
As well as dirty power lines, the outside noise seriously effect the backround blackness of sound. This noise can not be heared directly so we don't mind so much but if you check with dB meter in the night and afternoon, it is really so much. Also, I compared many power cables and I made some myself, eventually I found a magic power cord, sidewinder (www.powersnakes.com) This cable makes unbelieve differency on smoothness and blackness of the soundstage, give a try you will seee immediately (You may order long and cut to two or three without any problem)
You might also consider the difference in your ear's sensitivity at those extreme times of the day. Your ears will naturally become less sensitive when the environment is noisy (loud). And then at night, or early morning, they can pick up the slightest details. Try wearing ear plugs for an hour or so, and then take them out to see what I mean.
Maybe that explains why my system always sounds so good. I wear earplugs 8 hours a day at work.
This is a very important topic. The fact that my system(s) sound better late at night, early in the day, and on Sunday has been bugging me for years. There seems to be a definite correlation between power usage on the line and sound quality. Sound is noticably better in the middle of the night when power usage is at a minimum. The problem is that no matter what power cords, power conditioners, filters, etc. I put in the system, some of which do improve the sound, this problem never goes away. Is there a solution?