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

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.

I agree with mrdecibel, there are generally lots of switches with the audio signal going through them in older gear. There are some issues with that.
Apart from the extra wiring needed to get the signal from the circuit board to a front-mounted switch, which can make the amplifier cramped inside, there are contact issues to talk about.
The big manufacturers generally used silver plated switch contacts which have low resistance and thus good sound quality when functioning correctly. But being silver they tarnish and lose conductivity over time. Without operating the switch occasionally to get the contacts to scrape off the tarnish, bad electrical contact develops and very high distortion or non-function can result.
Silver plated contacts are generally best used for power circuit switches as they dissipate less power at higher current, and the small arc caused by switching high current ‘wets’ the contact and keeps it functioning well, until it eventually wears out. Then you get pitted contacts that can burn, and it’s time to replace the switch.
Gold plated switches don’t tarnish and have very stable contact over time, and are more expensive due to the higher cost of gold. But they have higher contact resistance and because gold is a soft metal, using the switch a lot will wear it out faster than a silver plated contact switch. The small arc caused by switching power with a gold plated switch will burn the contact much faster, and so they are used for low current signal switches.
So there are benefits and problems with both switch types. Today’s equipment tries to sidestep that with solid state switches made up of transistors (generally mosfets) that have no tarnishing or wear from operating them. So the reliability of modern techniques is far better when it comes to switches. Unfortunately mosfets used as switches have a lot higher, and non-linear resistance than a mechanical switch and so some high end manufacturers still use mechanical switches for routing audio signals. But they need to be maintained by servicing over time.
Thus older equipment can be good but service those switches!
Poweramps usually use relays for speaker protection and they need to be maintained in good condition, as they have a critical role. They need low resistance for speaker currents, but there is no arc when it operates as the audio power is at zero when it does turn on. So especially for these relays, the contacts need to be kept in good order for low distortion. Older equipment used speaker relays by default. Newer equipment can use electronic techniques instead of relays so the reliability is greatly improved.
Transformer coupled tube amps don’t need output relays as the output transformer itself isolates the speaker from the dangerous voltages power tubes run on.
In my own preamps I’ve resorted to using mechanical gold plated switches for good SQ, but when they wear out I’ve made the layout so they can be replaced reasonably easily, and ready for the next decade of use.

Just keep a spray can of switch contact cleaner handy when you notice a switch giving issues.

Echolane, +1 for moving the switches regularly to keep them working well.
I help maintain a studio mixing console with hundreds of mechanical switches. The advice given in the service manual is to do just that, operate them regularly to keep tarnishing to a minimum. Some of the switches are so stubbornly tarnished though that I have to disassemble the switch and scrape the delicate contacts clean manually. That’s from years of not moving them enough previously.