The MXL 603S is SPECTACULAR on accoustic guitar. Despite the fact that they are bit gelatinous on transients, the guitar sound can't be beat. I like the rounder, bass-heavy sound. They have a lot of push. I can only desribe the sound as "serious and substantial".
I believe Micheal Joly offers a great mod for the 603 @ OktavaMod. I have not heard it myself so cannot comment. See here:
OktavaMod - Shop