It is very hard nowadays to get a bargain deal on any of the good vintage receivers. The incredibly strong resurgence of vinyl (propelled in large part by "trendies" who simply think the gear is "cool, retro and fun to fool with") is driving prices up / availability down compared to even a few years ago. This wouldn't be as big of a problem if old age wasn't hitting these receivers like a tornado: many of the more affordable ones need repairs, yet good luck finding a local repair shop that knows how to fix them. Some parts, esp Pioneer amp bits, are in very short supply. Shop carefully.
IMO that Pioneer SX-770 is a bit too old to risk $125: most Japanese gear made before 1972 is best left to the collectors (unless you're the original owner). I owned a Pioneer SX-626 for a few years: really nice receiver with rich sound, a bit "darker" sounding than most receivers of the period. A good match for speakers with weak-ish bass response. The 626 is one of handful of "cult" blue-dial Pioneer models some people go out of their way to track down: I can't remember why now (something about the amp/preamp design).
Everyone should own a Marantz at least once, but their iconic appearance is highly sought after, making them hard to find at realistic prices. I owned a beautifully restored 2270 for three years, but sold it when I got a ridiculously high offer. I still kinda regret letting it go, but I never cared for the sound quality- way too dark (darker than my Pioneer SX-626).
I've had Sansui integrated amp/tuner combos, but not any of their receivers. The separates were quite nice, esp the AU-517 amp.
Strangely, the best-sounding receiver I've ever owned was also the cheapest: Harman Kardon HK-330c. Something about its amp/preamp creates magic with pretty much any type of speakers or input sources. I think it sounds even better than the highly-touted "twin power" models that were above it in the HK lineup. The 330c is fairly easy to find at reasonable cost, as it was popular in its day and many still survive. Unfortunately, all HK gear made before 1980 suffers from atrociously poor pushbutton switches that corrode over the years, becoming noisy then totally dysfunctional. No amount of deoxit can fix them once they deteriorate, and no modern replacements seem to be available.
If you see a nice HK, be sure to fully check the Tape Monitor, Mono, Contour, and Speaker 1-2 buttons- loud noises or a dropped channel are bad signs. Also watch out for loose connections / weak solder in the Phono inputs. Worth the hassle for the sound quality, tho: my unrestored ca1975 HK-330c bought off eBay for $49 beats my $900 Creek integrated amp.