I assume you're using the Digigram VX440 as your ADC? When you switched L/R mics, cables, preamp inputs, preamp outputs, you switched each individually and independently before testing, right? I.e. switch mics, test. Switch cables, test. Switch preamp inputs, etc. Instead of Switch mics, cable, and preamp inputs then test.
Sooo...what happened when you switched:
Mics?
Cables?
Preamp inputs?
Preamp outputs?
Digigram inputs?