"FWIW I have a little Technics receiver that the manual says is 40 Watts / channel. When I ran it up it sure SOUNDED like more than 40 Watts. When I measured the power supply Voltage of +/- 42 I had my answer."

Yup, 42 volt rails will give you a hair over 100 WPC, as long as they maintain 42 volts. But you do have it for a matter of milliseconds. that is called dynamic headroom. Some manufacturers gave that rating and it was based aon a short term bust of a sine wave, specified in milliseconds. However there is another factor about these "Old Marantz Watts" or OMWs as I like to call them sometimes.

The older amp topologies and the devices available limited them somewhat because of non-linearities. These happened worst when you got near clipping. Bottom line, your 40 WPC Tecnics or whater really IS about 75 WPC, but at like 2 or 3 % THD.

Later, they made really good square waves. By that I mean they got the linearity closer to the rails and that is why the older stuff sounds better. A 40 WPC amp now might only really be 50 WPC, because they could keep it linear though more of its high power range. That's why if I still had my old Marantz I could sell it for a grand, which I do believe is more than it cost new. (but not by all that much)

But yes, when you get older amps and receivers, don't be surprised if they even have double the power claimed. That would be 3 dB, that is all. The ones, maybe I can find some, that gave dynamic and clipping headroom specs could showya. If it has 1.8 dB clipping headroom and then on top of that 2 dB dynamic headroom, that amp is going to sound louder than its RMS power rating would allude to.

Only thing you can do is crank it up and enjoy, and hope your speakers can handle it. You really can't sue them for putting too much power in the amp.