You can't go wrong with the M and the MX series Yamaha amplifiers. They are built like a tank and can run for hours at maximum levels. Like most power amps they do put off some heat so I would highly recommend running a cooling fan system with your amplifier. This will help you and your amplifier have a long life together. Yamaha amplifiers are like a old Harley Davidson motorcycle, they will run forever if you take care of them and they will hold their value. They definitely give you the most bang for the buck.
I received my Yamaha amplifier packed as though it was coming in for reentry from an Apollo moon rocket. About 5 layers of bubble wrap as well as crushed paper surrounding it. The M45 amplifier looks almost if not as good as new, especially for something made in the 80s. I connected to my system and everything works great. No problems. I'm totally happy with my purchase. Yamaha amplifiers from this era are a rare and great find. The sound quality is unsurpassed. It will even operate in class A which is as good as it gets. I paid a fraction of what this amplifier would cost to buy these days. It drives my 2 Polk towers better than I expected.
I purchased this amp to run a 15" DBX subwoofer. This 125 watt amp run at levels set to mid range gives my system just the right punch necessary for all my audio needs, from rock to classical to movies, etc. I've had Yamaha natural sound receivers/amps since the 80's, and have yet to be disappointed in their quality.