VIntage Mark Levinson-ML3 verses no 20's

I am in need of some high current type amps that have a lot of amperage, like Levinson's gear but as well my hard to drive speakers need amps then can drop down to 2ohms! Can anyone tell me what the main differences are between say the ML3's and the 20's,apart from the obvious that 1 is a stereo amp, was thinking ML 2's but could use more of the wattage. As well buying amps that old are the parts available to service them etc