some 70's and 80's receivers seem to be popular

Is this just nostalgia? I thought the concensus was that even the best solid state during that era was harsh and hard to listen to for long. That is certainly my memory. I almost quit listening to the stereo back then, until I got tube amplification and realized what the problem had been (well, having been suckered into switching to cd's was also part of it). But now some of that equipment seems to attract fans and bidders. Are they just dreaming?
tandberg-marantz-mac-revox-sansui-luxman-yamaha etc all still sound awfully good, and still kick ass for the money. the phono sections in many are as good as many new high end phono stages. they are popular because they deliver.
That harsh, bright and "listener's fatigue syndrome" effect came about when the Japanese starting using lots of negative feedback to further lower the distortion figures. It seems that during the 70's and 80's there was a competitive factor involved that made some manufacturers strive for "who had the best specs", rather than "who had the best sound". I still have a mid-70's Harmon Kardon 730 that is very musical for its time, though. I recall selling an early 70's Sherwood receiver to buy the newest and latest receiver design from Technics and regretted it immediately when I got it. The sound was like beams of lazer piercing my eardrums. Ouch! I couldn't get rid of it fast enough.