I am trying to remix a song by radiohead and have the actual stems for the song as seperate midi tracks. Some I want to change, but some I want to leave exactly as they are.

When I import them it seems I have to play them through one of the samplers which means assigning a synth sound to it and ending up with a sound different from the original. How do I avoid this and actually get it to sound as it would in its original wav format?

This is impossible, because MIDI data only contain notes and control information. They do not contain no sounds or any information of the sound characteristics.

Yeah, you'd need to get actual wav files of the song, you can't get individual tracks like they are in the original song, patched and everything, unless you contact radiohead and they give it to you. which ain't going to happen. Maybe this new melodyne thing can do this.

__________________
...

RADIOHEAD

Which song is it?
Post the midi data somewhere for me.