El cheapo sleeper amps....

I have an el cheapo intergrated amp non-es Sony from the late 80s-early 90s era and Im puzzled why this relatively modest amp is such a stellar performer...and the phono stage is above average as well...any other sleeper el cheapo amps that out perform their modest intentions?
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Sleeper as in BPC? NAD stuff always impressed me. I've always liked the H/K twin offerings from the 70's in their price range.
I have been enjoying the heck out of a Heathkit sa3 integrated c.1960. Cost me a whopping $160. That's less than $60 per quality watt! Old school here. Flash forward 20 years, same holds true. Little $$+ big enjoyment= Smarts!! Enjoy our giving of Thanks this day!!
Why would you be surprised your Sony sounds so good? Sony is the company that made Marantz popular with their solid state Receivers, Tuners, amplifiers and preamps in the 1970s.

There are to many sleeper amps to mention all of them.
How did Sony make Marantz popular?
As far as Sony making a great sounding integrated...

Even a broken clock is right twice a day.

Not doubting it sounds good, just saying.
Absolutely. Refurbished Scott, Dynaco, Eico, Heathkit, Knight, Hammond organ, etc. vintage tube amps.You don't have to turn it up to loud levels to hear full sound and decent bass. Tubes are the low cost way to get harmonics/which IMHO is the heart of music. Autospec and Cjkinpa do great jobs and I can recommend them both highly.
Luxman L-3,L-5. Low powered but sweet.
My favorite cheap old solid state is a Marantz 1060. For tubes, a Dynakit SCA 35. I have owned and used several of each and keep running into them still working and sounding better than most of the competition.
"How did Sony make Marantz popular?"
I worked for Sony/Superscope when it bought Marantz and launched the Japanese receivers made by Standard Radio. Receivers were the coming thing in audio products at that time. The American make Marantz receiver was manufactured in NY and sold for around eight hundred dollars with about forty five watts per channel (as I recall). The Japanese model 2070 sold for four hundred dollars, had 75 watts, sounded better, was more reliable, and could drive almost any speaker of the day to live volume levels. The other Marantz receivers were OK, but the 2070 was hands down the best deal of the time and gave Marantz had its first mass market success.