You might check to be certain that the house system doesn't have some sort of exciter or enhancer and is using too much of it. As in life, too much excitement can cause problems.
Seriously,I have seen an aural exciter turned up too high cause exactly the systems you describe.
If not that, it must be something in the e.q. of one of the mixers. Certain frequencies are being over amplified, or cut too much.
I'll bet if you look at the e.q. sliders on the main mixer, it looks like a big smile. Modern sound equipment rarely needs that curve any more, but a lot of sound men automatically set it that way anyway, because that's the way they've always done it.
Don