El cheapo sleeper amps....

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.