There is more that's obvious besides one being stereo and the other mono. The No. 20's are class A amplifiers and the ML 3 is AB. Mark Levinson amplifiers are known for a warm sound and the ML 3 is even warmer yet. The class A No. 20 has a clarity and transparency not found in the ML 3. Service may be difficult for either amplifier if you want it restored to exact original sonics and specs. Both are best if used with the matching Mark Levinson preamp and there is always the issue of camacs.
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