How do you avoid comb filtering effects where there are singers or instruments with microphones spaced closely together?

I did some testing of various Oh placements. My room has a low ceiling 6'3 and I find I'm getting some kind of weird comb filtering if I set up OH's to close to the ceiling even though it's completely dead, it's insulation with fabric over it.. not a deliberate treatment setup. Here's some examples of random playing.

It appears as though I've run out of mp3 power.