As Daniel mentioned in https://bugs.launchpad.net/mixxx/+bug/1440675/comments/3 , a user did not understand why they got worse sound quality from Mixxx than Virtual DJ when the problem was that they improperly adjusted the gain control on Mixxx. Mixxx should be designed to minimize the possibility of this confusion.
Although the master and headphone "volume" controls were renamed to gain controls rather than volume (see bug 1306253), many users do not understand the difference and users who will be upgrading from 1.11 likely won't notice the change easily. The tooltips for the gain knobs and the manual should tell users to adjust hardware volume controls before software gain controls. The manual should explain why this is the proper way to adjust volume and how to do it in different cases (sound cards with knobs, sound cards controlled by vendor provided software, alsamixer). The manual should also advise users to check the input/output mix knob on sound cards designed for recording (as I had to learn the hard way).
Perhaps part of the solution to this could be separating the master gain, headphone gain, headphone mix, and balance controls from the main mixer section in each skin like how Deere presently is. Perhaps this section could be hidden by default and users would need to toggle a button to show it. If these controls were not glaringly obvious on the screen, I think many users would try to adjust their hardware volume controls before adjusting the software gain control that is right in the middle of the screen. If users are confused because they can't find the software gain controls, they may be more likely to refer to the manual which would have an explanation of how to properly adjust levels. Looking at screenshots of Virtual DJ, many of them do not show the master gain controls on screen. Also, Traktor separates the master gain controls from the mixer section.
Related bugs:
Bug 1440443
Bug 1440675
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