Thanks, as I replied in the other thread,I believe what you are hearing is the result of high output impedance of your headphone outlet, causing a negative damping factor with the Grado's lowish input impedance. Try your headphones with a modern headphone amp, or even your mobile phone. If the sound changes drastically, you'll have your answer.
"As is the case with most budget amps, the headphone jack takes its feed directly from the power amplifier, rather than having its own dedicated circuit."
https://www.audioappraisal.com/yamaha-as201-integrated-amplifier-review/
I used it with an old Sony D-EJ751 PCDP and, surprisingly, it sounds great!
So I think you are right, must be a problem with the Yamaha amp and, as for the laptop and the smartphone, maybe I'd need a DAC or so...
But, for now, at least I know I can use it with the discman. What a discovery!