Your original statement is absolutist and deserved to be treated as such. You are now changing your original statement. If you wish to make absolutist statements and declare them as fact then best to think them through and correctly phrase them first time around. Otherwise you will appear as, and be treated as, a troll.
Btw should the amp be designed to be unconditionally unaffected by "normal variations of mains supply" or just the ones within the supplier stated parameters? The two are not always the same. There's also the issue that what is "normal" has changed somewhat over time. There's a lot more shit flying around these days than say pre 1975 (selected as a point in time before which there was very little in the way of microprocessors etc sat on the mains and polluting it with elements outside of spec).