The OP asks about "sound quality". That means perceivable (by a human set of ears), differences between older amplifiers and new amplifiers.
In general, well designed high fidelity amplifiers have been virtually indistinguishable in listening tests, from one another, for at least 50 years. That is, similarly capable products. Apples to apples comparisons. No point picking a $100 Fosi and comparing it to a TOTL Accuphase is there?
Of course we can measure differences between all amplifiers, no two will exhibit identical performance, but it comes down to where you want to draw a line and say you've reached your own personal nirvana. For some people, it's a constant journey, others are satisfied at step one. I climbed my own personal audiophile ladder with new gear, while at the same time picking up and restoring gear others had cast aside, traded-in or otherwise passed on. Back then, I had to buy the best gear new to outperform the restored middle of the range gear from 20 years earlier.
For me, there's no current model amplifier from any manufacturer I'm itching to buy. Not even one. But if I saw a vintage classic at a garage sale sitting unloved in a dusty corner, it'd come home with me. But my passion is not just the "sound quality", it is the build
For someone starting in HiFi today, I'd advise not going down the vintage path. Much of it is now reaching the age where it's not just components that need replacing, it's cosmetic, structural and otherwise difficult to obtain parts you will need to find. If you like ongoing projects, sure, go for it, but if you want simple plug, play and forget buy new.