Official comment
@Martin > As @Theo outlines below, the line inputs on the DDJ-1000 are fairly comparable to our DJMs; if the line device you're connecting to one of the DDJ-1000 inputs doesn't provide an adequate signal, turning the trim/gain to full may still not adequately boost the signal.
In my testing, I've found the levels to be suitable with an iPad, iPhone, CDJ, XDJ, and many other devices with line-out or headphone-out capability. For those where the output is headphones, it's important to turn the volume to the maximum, and ensure you don't have any volume limiting enabled in the preferences.