Mid-Fi: like run of the mill, like Sony.
Hi-Fi: a step above in quality of build and/or sound.
I have a mid-fi '89 Sony amp that sounds really nice...but it's mid-fi. But it does sound good! Really good.
I have a '87 Sansui amp that sounds 'outstanding'. It's Hi-Fi. It's also a very high quality build.
Hi-Fi has in-depth, deep, real, natural, honest, live-'alive' sound. When you hear Hi-Fi it's like: "oh yeah...that's it!!" You know it immediately. Your there. You got it. No messing around. It's the real thing. It just sounds 'right'... No thinking involved. It works.
That's it...like that.
I've had a few Mid-Fi stereos that sounded outrageous. Would they be Hi-Fi? Maybe. I would think so. Maybe not Hi-Fi in 'quality of build' but certainly Hi-Fi in quality of sound quality.
If it sounds, and 'feels', most excellent to you...it is most excellent.
I've got an old Hitachi mini-component system, 15 watts, that I bought back in the mid '80sand it sounds REAL! Just outstanding. Like you ARE THERE. Would that be Hi-Fi?
'High End'...big bucks, for better or for worse. Hopefully for the better.
Hi-Fi is all in the ear of the listener. if it's right, it's right. And if it's wrong, it's wrong.