Muddy sound is likely to be caused by one of two things: dirty heads, or improper head azimuth alignment. Dirty heads are an easy problem to eliminate. Clean the heads on both machines, and then see which one gives you better sound.
For head alignment, you have a classic case of which deck is right, which is wrong. The best way to answer that is to compare both decks to an objective standard: an alignment tape. Which machine does the alignment tape look or sound worse on? That's the one with the Azimuth problem.
But if the one that's out of azimuth is the machine that the original tapes were made on, make sure you transfer any sections you need to use from the tapes that were made before you adjust that machine to correct the error in alignment. You want to play those back with the deck that matches the incorrect azimuth alignment that existed when the recordings were made.
See Azimuth for more information.