Yes, people have claimed that various models sound better than others. There are a lot of claims made online, and I frankly don't believe quite a few of them.
When the SX-1010 was being phased out, and the SX-xx50 series was introduced, how many people
at that time wrote that the new series sounded worse? If the SX-1010 really sounds better, why is it that
NO ONE seems to have noticed at the time when people could compare new units, side by side, in the stores (such stores, by the way, were common in the 1970's)? So, no, I don't believe such claims that are made now.
But you are going beyond what I state when you state:
Based on your logic, the SX-1980 would have had better sound quality than the SX-1280...
I doubt that, in most situations, that most people would be able to distinguish between them in a properly conducted double blind test. Of course, if the SX-1280 were driven to clipping and the SX-1980 were not, then it should be easy enough to distinguish between them (assuming, of course, that one has not gone deaf from the excessive volume that would likely be involved in driving the SX-1280 into clipping). There also
could be a difference heard if the listening were done with the tuner, in an unusual case in which the SX-1280 had a problem and the SX-1980 did not (the SX-1980 had a very, very slightly better tuner), though most times, both would likely sound the same in practice.
As for the late 70's having "a general cheapening of parts", it is certainly true that the quality of construction went down after the SX-xx50 series; the SX-xx80 series, though well-constructed, is a step down in that regard. One need not even open the cases to see this; one can look at the wrap-around heat sinks on the SX-1250 and compare with the SX-1280 and SX-1980 and see that they did not bother with making the heat sinks wrap around any longer; it was cheaper to have the heat sinks as was done on those newer units. The heat sinks, of course, are not a problem with the sound, nor even with how well it is otherwise constructed, but it does show quite clearly that they were looking to save money on the construction of the newer units. But construction quality is not the same as sound quality, though many people seem confused by this, which explains some of the fanaticism about McIntosh gear. I have read, on several occasions, that once someone tries a McIntosh piece of equipment, they never go back to other stuff. That, of course, is the purest BS nonsense. I have owned McIntosh gear, and I got rid of it and do not own any of it at this time. Nor will I ever own any of it ever again, unless I get something at significantly less than "fair market value." Sure, it is well-constructed, but that does not make it sound better.
Of course, some of what is going on when people listen to things today can be explained by the fact that 1970's gear is all over 40 years old, and so there is a very good chance that some random piece that one gets ahold of may not be functioning up to specifications. A properly functioning SX-450 may well sound better than
any higher model that has serious problems.