Short answer: the MXL 600 sucks
The MXL 603 is a nice detailed microphone, especially good for acoustic instruments like guitar, mandolin, etc.... The MXL 600 is more expensive and yet, paradoxically, the cheaper MXL 603 is a much better microphone. I was fortunate to find a local store that took my MXL 600 in trade for an MXL 603S, and I felt bad for such an unfair trade!
The MXL 600 embodies the sound of mud.