Could be anything. Could be the amp topology. Maybe the substrate. Might be the surrounding components. Might even be the music or the moods.When you compare 'em, please let us know.
I remember the Yamaha receiver in the 1980's sounded really great. I was impressed with it in several ways. I don't know how it compares with some of the gear I have now, but it sure made an impression on me an many others that heard it. I ran it on seven π Speakers with all JBL Professional Series drivers.
To tell the truth, the inexpensive Harman Kardon HK 3470 receiver I picked up on Crutchfield a year ago sounds pretty good. I bought it to see how an entry level home theater would sound with two π's and tower two π's. I wanted solid state with about 100 watts per channel for this particular setup. That way I could compare tubes and solid state in the price range that most would probably use with one π's and two π's. I've never opened it to see if it uses a chip amp or discrete components, but more than likely, it's a chip. Maybe a National chip, just like the gainclones. I dunno. But it sounds pretty good; Suprising, really.
Wayne