The better ones were very good indeed, in many ways better than modern entries of the same class. However, be aware that not all brands and not all models were. Like today, there are better and worse. Like today, the worse were mostly sold by department stores and tinker shops, the better ones by audio salons and specialty distributors. An example refurb might be the cost to refurb a Sansui AU 717. A standard rebuilder online might quote a flat rate of around 500 dollars some less, some more. Recallibrations would be included, and all caps replaced, board cleaned, pots cleaned or replaced, a fairly standard set of parts used that generally age poorly, whether they have gone wrong yet or not. However, this is an integrated amp not including a tuner. the matching supertuners are also available in the Vintage marketplace however.
These units were heavier, but by reason of having beefy power supplies and good heat sinking. You will find this no different in any reputably built modern AB class amp/receiver. Circuit boards, however populated otherwise, really don't add much to product weight. Saying of a receiver, well its light, is like saying of a beater car, well, its blue. However the lack of weight does mean you are not getting a unit with a large power supply. I.E. the amp-on-a-chip junk, which don't require them, and are therefore cheap, class D or worse amps. While they are fine choices for PC sound, or car audio, these are not comparable to units made of discrete componentry in sound or quality, quite yet. Some are audibly close.
Just remember that these vintage units do not have HDMI inputs, and cannot in the main, handle any sort of digital decoding on their own, Such stuff was not around then.
The Sansui unit you mentioned had a SN ratio of about 87, 300 watts per channel of underrated RMS wattage at less than o.oo7 percent harmonic distortion, and probably wouldn't fit on a standard shelf. It was really a preamp/tuner bolted (literally) onto a separate dual monobloc Amplifier. Its specs were impeccable, and you can pick one up in good condition for about 8,000 to 12,000 dollars when you find them. Only about a thousand were ever made.