Sorry no one has posted. I'm dealing with the same thing at a church I'm installing at. They are 4 blocks from a 10kwatt AM station.
How long is the substitute snake? I've been told that Mackie's have trouble with their mic pre-amps and lengths of cable over 75'.
Here are a few things to try.
1: don't use phantom power.
2: has there been any water damage to the boxes and cable runs in the floor? Sounds like the shielding has been compromised.
3: can you pull some new cable through the floor with the stage boxes?
4: conduit? any in the floor?
5: Isolation transformers?
It's been since February, so you probably had someone come in and help out.
Fixed & Mobile Studios
Pro Sound Sales/Installation
MacXPRT Network member