Vintage gear is a real gamble, not because of the design, but because it is 40 years old and you have little idea how hard it's life has been. A 250 WRMS class A amp would see very high temperatures for sure, but it may have been paired with some pretty efficient speakers.
My best advice is to pull them out. Since the amp is the item you are most interested (and most subject to heat) pop the top and take a close look. This is mostly obvious stuff (I'm no EE) but look for signs of heat: Discoloration, distortion of component surfaces, leakage at the bottoms of the capacitors, etc. Also look for film or residue (the tar from being exposed to heavy smoke which will lower the efficiency of heat removal -we are talking 70's). Vacuum and/or blow out the dust as best you can, then plug them in.
You'll want to take pictures for ebay or for us if there are questionable situations.
I have a vintage unit (Sansui AU-717) which has had more problems resultant from the refurb efforts (some guy north of Knoxville that speciaized in Sansui Refurbs) than the original gear, so if you decide to keep/repair, research this carefully!
I still have mine because we've been through a lot of life together and I tend to personify much of my "stuff". It is sentimental, not functional.
I do not believe the Pioneer can be considered a better design than the ATI built Monolith. I would hope we have learned some things over those years. However, I do not believe the Pioneer would be audibly deficient in any way. I would look for those improvements to translate to better protection circuits and higher reliability of construction (for example, I might guess the thickness of a soldier joint to allow it to flex from heat without breaking has been optimized). ATI has been building the basic Monolith design for at least a decade.
I believe the value of vintage gear is more emotion driven than practical. That amp was made in the "Golden Age of Audio". In 1977, my home town of 100,680 had 4 high-end audio stores. The local Singer (Sewing Machine) store also sold Magnivox, Fischer, Marantz, and Superscope!
Music was being revolutionized by groups like The Beatles, Pink Floyd, ELP, David Bowie, Simon & Garfunkle, Chicago, Beachboys, Yes, Jethro Tull, CSN&Y, Queen, Elton John, etc. (which may have been further promoted by the ready ability to make high quality recordings and play them back at similar quality in interested middle class homes).
It was a perfect storm for music/audio!
For a geeky HS student like me, time spent listening in these stores and collecting literature on these audio marvels was an obsession like later generation's Pokemon cards.
It is easy for me to remember my first listens to high quality speakers of that time as being the "best speakers I have heard in my life"! Are they? Of course not. However, they could be so much better than anything I had commonly heard, that the excitement and experience far surpassed all later experiences!
So think of it as the guy who couldn't afford, but lusted after a 69 Convertible Mustang when he was 18, so later when he has money, he is happy to buy and recondition that wonderful vehicle!
I infer you are younger and the (emotional) value of those vintage units is mostly lost on you, so get out of the way and pass them on to someone who will summon memories of times past and smile every time they see it in their house!
Maybe I am "over-romanticizing" this. Certainly there are many of today's youth that fawn over Vinyl, so there is a group of younger people who also might covet the vintage gear, but unless you are one of them, I say "keep moving, nothing to see here"
