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

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.

I had commented on this question in the OP’s other recent thread. An excerpt from my comments, with a few words shown in brackets added by me now:

Regarding comparisons of vintage vs. recent equipment, a seemingly obvious point but one which often seems to not be taken into account in discussions of this subject is that comparisons should be based on similar present day prices. For example AustinStereo pointed out above that a restored SX-1050 is worth around $1000. So a fair comparison between that component and modern ones should be to a modern component (or a combination of components) providing preamp, power amp, [moving magnet phono stage], and tuner functionality, with comparable maximum power capability, for a total of around $1000.

Regarding some of the other points that have been made, it is certainly true that "distortion wars" occurred during the 1970s, in which feedback was applied in a heavy-handed manner to produce the lowest possible Total Harmonic Distortion numbers. With the consequent adverse effects on Transient Intermodulation Distortion and increased emphasis of distortion components that are the most objectionable not yet being generally recognized....

... My own preference among vintage components is for well restored tube equipment from the 1950s and 1960s, a lot of which can provide very good sonics and great value relative to present day price points.

Regards,
-- Al