In order for our mastering engineer to properly master your file, we ask that you deliver it around -6db and with no compression / limiting. By following those steps you help our creative process as we aim to give you a solid master. Below you will find some screenshots of wav forms. The first is of a correctly mixed down wav form with the other two displaying mixed down songs that were clipping due to high volume, compression / limiting.
More information on what our in-house mastering engineer does with your premasters can be found here - http://symphonicdistribution.com/comprehensive-mastering/