Changes in voltage at the wall socket affect the amp, for instance my amp sounds bad at about 6pm when everyone is cooking or watching tv, basically this relates to the B+ voltage (the voltage to the plates from the power transformer) which is the voltage u figure your negative bias voltage against, if your amp is showing a different B+ due to the wall socket changing voltage you will have a colder or hotter bias because this bias is controlled by a variable resistor that is "fixed" at a certain point it can change the tone of the amp drastically.
especially if your biased just out of cross over distortion when the wall socket is high and when it drops low your going into cross over (not a nice sound).
Other factors that can come into play are the heaters, these provide a small amount of AC to each tube to heat the tube, this voltage shouldn't change with the wall socket but what will happen is after 15/20mins of playing your tubes will be hot and this affects the tube's plate dissaption which can also change the tone of the amp slightly not too drastic tends just to warm up the tone a bit after a while because the tubes are hotter.
tubes also have a life span so effectively they're always one step closer to death this is a factor also and should be considered if you've had some tubes in there for years, i've got some old Mazda 12AX7 Silver Plates in my '70 Marshall that still test good, but the new chinese/russian tubes don't have that kind of life span
