Dear Martijn,

not easy to judge, but recorders and flutes playing close together (space- and frequencywise) are one of the best sound sources to demonstrate "difference frequency distortion", i.e. non-harmonic components generated by non-linearities in the audio chain. Difficult to localize if it is the microphones, the preamp, the loudspeakers, the air (! yes, when it gets VERY loud) or the ears which produce the distortion.
One possible solution for the difference in the recordings could be:
- AB omnis pick up BOTH recorder signals in each channel. Such a channel can then produce difference tones.

- DIN/XY/ORTF mostly pick up just one instrument, with the other one much attenuated by the polar pattern of the mic. This leads to less difference tones caused by non-linearities.

Best regards,
Martin Schneider / Neumann Mic. Development