Good advice. Hiss is never a good thing. He should maybe look into a budget headphone amp and budget source like the Sansa Clip+ which has expandable memory or a Nano. I know the LD tube amps MKII+?, MKIII have 3 gain settings which can be set to match the the impedance/sensitivity of the headphones. Might be a good amp to power the 650 with? Source material is very important as well, was the hiss present at all times, was it still there when the MD was connected to an amp?

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne