El cheapo sleeper amps....

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.

Rrog,
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.