To get right to the OP’s point about "specs": The attempt to assign a number to audio performance is often a futile attempt to arrive at something meaningful. Those numbers usually fell into various measurements of distortion and it seemed inconsistent that more expensive, more exotic gear would publish inferior specs compared to the receivers. We later learned that the popular standards of measurements were less meaningful than other "below the radar" specs, or performance parameters that were not published at all. To take some anxiety away from potential customers hyperventilating over "specs", I used to tell them that there were only 2 numbers that mattered: The price. Can they afford it? And, the physical measurements. Will it fit in the room, cabinet, etc? I understand this is a oversimplification, but did put them at ease to open up a meaningful conversation related to what mattered most to them.
Related to the question: "Did the old receivers sound good?", my answer is an emphatic "YES!!!" If we’re refering to the delivering a musically satisfying emotioinal experience, I think I can site hundreds of examples -- from a customer index cards we collected from the 70s to validate this claim.
If receivers from the 70’s were a good example of "good", then the integrated amps from the 70’s were "better" and separates were "best".
But, time marches on. About 10 years later, Nakamichi teamed up with Nelson Pass and incorporated Statis technology in their receivers. With zero negative feedback and other innovations, these eclipsed earlier designs IMHO. I still have a TA2 and TA4 in my collection of "excellent" examples of stereo receiver performance of the 1980s.
A study of the performance of the "big boy" receivers of the 70’s will find a couple performamce constraints that can be remedied. One easy & cheap. The other a little a little more complicated. When the preamp jumper is replaced with an "audio quality" short interconnect/jumper they really open up and become more detailed. Time: 15 seconds. Cost: about $40. The second constrant is the power cord. Even the best receivers (and most separate power amplifiers) of the day had a power cord attached that would not be suitable for a 2-slice toaster, muchless a piece of serious audio gear. Replacing these with modern "audiophile" power cords and upgraded "protective" device" pays major sonic dividents. We routinely do these "mods" in our shop, and I am often stunned at the sonic transformations. Yes, you get the expected improvements in detail, focus and dynamic contrast. But, the bigger surprise (to us anyway) are the changes in tonality, chord structure, warmth, balance, etc. More relaxed, fuller, and musically engaging. The power cord upgrade can be a minimally invasive removal of the factory installed cord and install the better cord in the same manner as OEM (maybe with a slight enlargment in input opening). Or, if the user is okay with sometinig a little more aggressive, cut an opening in the chassis and install a male EIC socket, replacing the internal high voltage wire with better cabling. This allows the customer to not only select their own power cord, but upgrade the cord in the future. Does this make an old receiver sound better than the latest technology? No. But, if they love their old receiver and want it to sound better it is a worthwhile investment.