Anyone done A / B listening to compare vintage stereo receivers with today’s Amps/Preamps?

I had the “ legendary” 70’s era MARANTZ 2270 receiver that has evolved over the years with many system incarnations, to my present 2-channel integrated amp audio systems (.... 3 of them ranging from $3K to $50K...) (I also still have an early 80’s system as my garage system as a 4th )

In brief, in all cases the current audio electronics product offerings audio performance clearly best the vintage receiver audio performance, with a performance improvement gap rising exponentially in lockstep with the price point strata.

Simply put, the technology, design, build and resulting audio output today is far superior today to 70’s era gear, notwithstanding that the latter has a certain nostalgia appeal to a certain segment of the audio enthusiast population.

The parts used today are simply on an entirely higher level of quality and performance. Just this fact alone means much has changed with amplifiers over the years. 

Things that have changed over the years are: solid state parts were designed as through-hole components, now mostly surface mount and integrated circuits. That makes the assembly a lot more compact and so eg. circuit board inductance reduces, allowing designs with lower distortion at high frequencies. Microprocessors have taken over duty from discrete circuits that monitor the safety of the circuit and your speakers. Resistors have lower noise and tighter tolerance. Capacitors have better etching techniques to make them both smaller and lower impedance. Remote control is common. Digital techniques to manage analog circuits are common, as is the digital format itself. The technology of tube equipment has benefitted mainly from better tolerance components and much better high voltage capacitors. Some tubes themselves are also now made using computer-guided precision, an interesting use of two seemingly opposing technologies. So the extremes of tube batch variations are much reduced.
But don’t discount the older stuff just yet, there were some cracking state of the art designs in the late 70s and 80s like Sansui and high end Onkyo who’s circuits would be very elaborate to build today. TBH I’m not familiar with Pioneers of that era though!
That turntables, for all their flaws, are still around and sounding better than ever tells you that just because technology is old, it doesn’t mean it’s not good anymore. There is also a trend towards reel to reel these days, old tech done with new techniques!
So there’s a general trend over the years to lower distortion which can manifest itself as transparency in the sound, and more stable circuitry.

Great designed new stuff is usually much better.