Hi,
I had this same problem running with Soundprofessional AT853 card and SP-SPSB-1.
First time I thought it was because of the config of the mics (were set X-Y), later I tought that the mics were "phase cancelled".
Finally I discovered that the wavs were somehow distorted (in bass).
Are you running 2-wire or 3-wire? Do you use some of Chris Church mod's? (4k7 resistor, 3-wire....)