I just submitted a bug report about Mixxx's lack of a limiter. I believe that there should be a setting to enable or disable limiting the master output of Mixxx so clipping / audio distortion could be avoided at high volumes. This would also make it easier to work with multiple amplifiers (Mixxx's master could be set at 100% and volume could be adjusted by remaining amp).
Any thoughts? See https://bugs.launchpad.net/mixxx/+bug/1009987
