Your observations of fluctuating mains voltage was one of the first areas I noticed years back that affected the sound of my system. I found that after 9.00PM onwards the noise floor dropped at a result of variations in the mains quality but I did not actually know this was a fact and had been known about until I started mentioning it on forums and asking friends if they had experienced the same phenomenon. I just attributed it to less traffic at night and the fact that my system had warmed up for 3 hours or so. Now I know that noise on the mains can be greatly reduced at night as the general neighbourhood starts switching off devices and less power is consumed on the grid resulting in better mains quality thus reducing noise floor in my system.
My experience with humidity is not as pronounced and obvious but on occasions when I have used my dehumidifier and got the humidity to below 60% I have noticed that it affects the sound of my system. Now I must also say I have noticed it sounds better in the winter rather than the summer and has done for years but I have not mentioned this. Humidity is often a fair bit lower in the winter than the summer especially when you consider our wet warm summer and that water moisture is held at a greater percentage in moist warm air rather than in cold dry air.
Using the dehumidifier has probably allowed me to obtain the same humidity in my listening room in the summer as I would have probably have had in the winter and as I have stated my system definitely sounds better as a consequence. It may well be down to the comfort factor but I am not so sure?
Main system : VPI Scout 1.1 / JMW 9T / 2M Black / Croft 25R+ / Croft 7 / Heco Celan GT 702
Second System : Goldring Lenco GL75 / AT95EX / Pioneer SX590 / Spendor SP2