How is Mid Fi defined?

a term perpetuated by salesman so they can sell hi fi IMO

Your system is mid fi if selling it during a time of financial hardship wouldn't make a significant difference. This definition is variable as well as money has different value to different people.

In my case, I doubt I could get enough money selling my entire system to really make a significant difference unless I was getting down to not being able to afford food. A few thousand dollars would be helpful in the immediate short term, but unless the actual problem was solved I would likely choose to skip that particular step and move on.

I've never understood people selling all their stuff. My brother-in-law sold his blu-ray player a while back and likely got $20 for it. Did it really make a real difference in his finances, I doubt it.

Mid Fi is apparent when you hear something demonstrably better by any and all standards, on a level you never thought possible. Until then, you simply won't know it until you here it.