Power tubes and certain long plate preamp & rectifier tubes are sensitive to vibrations and can go microphonic. Just maybe if the first generations of amps where SS. Heads may not have become common before the rack trend of the 80s. Heads in wooden cabinets may have never become common Having the amp in a separate box has definate advantage with tubes. If it's all transistors; in my mind we're down to convienance/portability, versatility, ascetic and the options available from manufacturers. I typically buy stacks because this stuff is expensive. If I can buy a piece at a time i will. I'm not wealthy and avoid using credit for non emergencies. Same strategy buying a sectional couch one piece at a time.