I can't tell you much about the older macs, but I do own a pair of 501 monoblocks. To say they are impressive, would be an understatement. I have also heard the 402 amp and was also very impressed. I chose the monoblocks over the 402 for the added degree of transparancy and palpability. While the difference is not huge or night and day, it is there. I have also heard a fair share of other SS amps out there, none touched me as much as the mac amps. Huge power reserves to drive any speaker from bookshelf to the most inneficient behemoths. Smooth, grain free, tube like midrange, delicate highs that actually make cymbals sound like brass and not breaking glass, very tuneful, musical bass. What else can you ask for?