i don't fully understand bricking the pre myself but if the spl coming in to the pre is very hot, the preamp circuitry can be overloaded...this is why the sony portables have problems when you run them mic-in...the pre stage overloads easily...when this is the case, if you apply too much gain, the pre overloads and you don't approach overs on the a/d, but it sounds like muffled clipping (it's really an awful sound). i think this might happen here because with the 20 db pad on the v3 engaged, he may have had to apply a lot of gain to get the levels where he wanted on the a/d...however, at peaks in the music, it was probably overloaded the pre-amp circuitry...i'm not exactly sure if this is correct, but it's my best guess.
Okay, so, potentially exposing my ignorance here...but I have a different understanding of brickwalling. My understanding of brickwalling: too hot a signal hitting the
input of the gain stage. What this means is adding gain after the initial input to the gain stage - as noted above, I
think - does not cause brickwalling. Adding or removing gain *after* the input to the gain stage won't solve the problem. Only attenuating the signal *before* it hits the input to the gain stage would solve the problem - no amount of gain or attenuation after this point would fix it.
All that said, this situation still sounds funny to me. The clicking/distortion starting at -6dB is indicative of brickwalling, but the V3 with the jumpers set to -20dB is capable of receiving a pro line level signal, which I
think should be far hotter than the mics can output, even if extremely sensitive as you say (no factual reason for me to think this, just gut feeling).
Have you had any problems with brickwalling running these same mics into other pre/ADCs in similar venues/bands/sound systems?
One suggestion: see if you can borrow a pair of attenuators to see if it solves the problem (which would surprise me at this point) - we're talking -12dB on the mics and -20dB on the V3, that's a fair amount of attenuation!
Do you have an alternative pair of mics you can try out in the field?
Have you swapped out the mic cables?