Older vintage amps does sound alot more different than modern amps, some even has frequencys to choose from in the highs and lows, not just a fixed value
I once read an article about the loudness button. It seems static but it follows a curve up till a volume at which there is enough presence from the volume itself and kind of transposes to "normal mode". Owners of bookshelf speakers really take advantage from it. But when owning large cabinet speakers the button doesn't add much value as the cabinet size shows its benefits without aids.