Updated: After another hour, the left channel became half as strong as the right, then harsh. Half of the amp became extremely hot and I shut it down. Combined with the noisy phono section, and the extraordinary complexity of the design, I decided to return it to whence it came, my dream of a twin powered Harman Kardon super receiver, unfulfilled. The HK twin powered series, particularly the 930 and 430 are loved by many, and considered to be among the best. The 930, at 45 watts/channel (albeit conservatively rated) is the most powerful. I bought this 900+ quadraphonic receiver with no intention of using it in 4 channel mode. It's rated (again conservatively) at 45 watts/channel in quad, 90 watts/channel in stereo, but as this thread from Quadraphonic Quad makes clear, it is capable of over a hundred. That makes it a rare beast, a twin powered HK super receiver.
After giving it a good cleaning, I've been listening for an hour, and FM and digital sources sound beautiful, with the clarity and authority that I expect from Harman Kardon. Unfortunately, the phono section is very noisy in one channel. My guess is that it's an easy fix, but considering the complexity of this machine, it could be a big job. I'm happy already, though.