Funny, we were just discussing this here this morning. Working on completing the rebuild / restoration of my second SX-1010, the best sounding model Pioneer receiver I've owned.
Having compared: SX-1980, SX-1080, SX-1250, SX-1050, SX-950, SX-1010, SA-9900, SA-9800, SA-9500II, SA-7100 and several smaller SX-xxx and SX-x80 I feel that the 1980 is great in the spec book, but doesn't have the power supply to maintain headroom (and thus its mediocre 4Ohm spec), plus I feel that (like the SA-9900/SA-9800 and to a lesser extent) the SX-1080 is shrill and likely has more of the harmonic distortion that is unpleasant causing the fatiguing sound.
The SX-xx50 receivers are flat and powerful, in my experience/opinion the SX-950 has more warmth and a more musical sound to it. The SX-1010 is IMO the best of the receivers because well, it just sounds best and tends to play well with any speakers I hook to it, never shrill.
Now you do realize that all of this is completely subjective? My wife doesn't like the same speakers I do although we both like (most of) the same music. I have several sets of speakers, but I don't think that I have any that are less than 89dB efficient, so I don't need a ton of power, and I listen mostly at lower SPLs. My room is medium-size with lots of soft surfaces, I listen to most types of music, like Ti domes and Heil tweeters, electrostats, so HF harmonics have nowhere to hide, always listen to my music flat, play vinyl as well as lossless FLAC, do most of my listening now to tube amps, currently not using a subwoofer in my main system, ...
If I were still running inefficient 4Ohm speakers that dipped below 2Ohms, ... none of the SX-xx80 would have cut it anyway, and the SX-1250 although better suited would still have fallen flat in high-dynamic music. If you're listening on nice 2-way ARs that only need 15watts, you might find that an SX-737 is the best thing you've ever heard, ... or in my experience the SA-7100 is a great low-watt integrated with exceptional character. Thus the difficulty in picking gear from a list.
I rediscovered my inner stereohead a few years ago with high-end modern gear, quickly started to acquire (lots of) mid to high-end vintage gear and restore it so that I could have the best vintage gear, A-B-ing ad-nauseam the restored vintage gear to decide which one I liked best, which to keep and which to pass on, looking for better and better (part of how I came to own the gear of which the above list is a very small sampling).
Eventually I realized I really don't care.
Not that I want to settle for crap, but that once you're enjoying what you have then you're just wasting your time with the opinions of others which is based on variables and tastes that might not match your own.
I do appreciate your quest for quantitative data comparing these models so that you can choose the best for you, but it isn't there. Although I've read others who also have had opportunity to compare the 950, 1050, 1250, and 1010 and it's a fairly common opinion that the 1010 is the best sounding of the bunch, and that the 950 is the best sounding of the xx50 series, I'm not sure I would have come to this conclusion in 1980 when I was listening to Pink Floyd THE WALL at infinity dB. I liked sharp, and the 1980 was the most brutal hard-edged sound out there (opinion), had something that I didn't feel that was there from the other SS Japanese manufacturers.
That is great right? Funny thing is, I regret selling my SX-1250 (and also the 1010) so having found a replacement SX-1010 I am watching for a nice 1250. Why no 950 then? Because it's a collection and I really have no desire to listen to any of them anymore, I have other equipment that IMO makes them all sound like boxes full of transistors.
Get what makes you happy. For some of us the visual presentation is worth as much as the musical presentation and it's hard to beat the silver-face SXs.