Part of the myriad of huge amplifiers Yamaha produced during the 1980s : well-built, powerful enough to drive a pair of Magnepans, looking in sync with the times (flashing red and black) and nearly indestructible.
Auto-Class A allows the amp to function for its first few Watts in full Class A and become a room heater along the way.
The ZDR system was applied here as well as a fully DC structure.
The three load switches are not reminiscent of Yamaha's 1980/82 "output impedance" feature but merely a way of adjusting the meters to the actual power being output.
If one uses fairly efficient loudspeaker, one can actually use three speaker pairs concommitantly - the M-80 is a real Kraft Paket !
Very successful market-wise as well : there are many many M-60, M-70 and M-80 around the world ; the M-40 is much more rare.
Unlike the previous C-70 & M-70 combo, the C-80 and M-80 were not released in Japan.