Without listening to the samples, my guess is that the mic capsules were overloading and could not handle the bass put out at the concert if the volume was as loud as you say it was. In that instance, a bass rolloff filter will not work because the distortion from the capsules would not be affected by the rolloff applied at the recorder. The only way around this, IMHO, is to purchase new microphones that are not as sensitive. If you record concerts that have that level of consistent SPL levels, you may want to check out the dpa 4061 microphones or something like that.