Theory is one thing. It does not always equate to practical experience and preference.
"don't feel it is possible to reach equivalent quality out of great headphones with a cheap amp"
I would have to say no to this: Adequate quality yes. Optimal quality no. Here the headphones would be held back and the amp would be the bottle neck.
"as medium headphones and a great amp"
Here you would be able to reach the full potential of the headphones. This is the most desired way IMO
Note: this is not specific to the MDs or the HE-500s. this is just in general.
Cool, totally agree with you.