"MDS-501 isn't really a portable, it's a deck"
Ah. Silly me, I'd forgotten... I have a big black Sony MD deck, but it's not a 501.

Strange that you have a computer sound input that needs more signal that a consumer tape or MD deck can provide.

How about getting a device like THIS which can take both analog and digital audio input?
One purchase (and cheap enough?) that should work with both your tape deck and your MD digital audio out.

Anahata