I'm not familiar with that particular amp but here are a couple guesses.
First off its not likely a FET or MOS-FET design. In the early 80's power FET's just weren't very common in consumer audio power amplifiers, especially a receiver. They were just starting to appear in higher end component power amps about that time. FET's were pretty common in small signal gain stages though. Also note that the transistor technology used really doesn't define the operating class or its quiescent idle current. It is possible that many MOS-FET designs ran higher idling (bias) currents but this may be due either to them aspiring to better sound or the better resistance to thermal runaway that MOS-FET's have compared to bipolar junction transistors.
In an audio power amplifier of any size true class "A" operation really doesn't exist. For reference Class A means that the output devices are conducting current continuously throughout both the positive and negative parts of the amplified signal waveform. This creates a LOT of heat even during times of zero signal.
A Class B design only has the output devices conducting when there a signal and there will be one device to handle each polarity of the waveform. A true Class B amp will run basically cold to the touch. Class B amps don't really exist in the audio world either due to linearity problems as the transistors are switching.
What most all amplifiers have is a blend of the two classes, often known as Class AB. In these amps there is an idling current so that each of the polarities of output devices will be conducting continuously, but only up to a certain level when they effectively get cut off. This gives the linearity advantages of a Class A design with the efficiency of Class B at higher outputs. At higher signal levels the distortions due to non-linearity are much smaller and less noticeable so its a reasonable compromise. Still the engineers play a lot with the idling currents so some amps may dissipate more or less heat idling then others.
As for the perceived audio qualities its not likely due to small changes in the operating point of the power amp. It could be due to small variations of frequency response or perhaps interactions with how the amplifier interacts with the complex impedance of your speakers that is different from other amps. Of course listening is a physco acoustic experience so one can't fully separate out what your brain wants you to hear. In other words audio perception is influenced by other factors, looks or preconceived ideas. If it weren't for this fact they hucksters that sell $1000 speaker cables would have to find another avenue to con people out of their money
Beware though at 30+ years of age, replacing electrolytic capacitors will soon be necessary. But, the bottom line is that you found a good component and it makes sound that pleases you. Enjoy your music!
