Offizieller Kommentar
The channel levels should all be equal - I typically recommend people aim for audio peaks at the first orange LED (which is different on some products, and in the case of the DDJ-400, doesn't indicate what level it represents). This means you will have to adjust the trim so the song peaks there each time you load a track - you can't just set it at 3 o'clock and expect every song has the same audio level.
Take the master level up to around the 2 o'clock position - this is approximately where the master and channel levels are going to be equal.
Your output should be at a nominal level and you should then adjust the input on the A&H such that the levels aren't too high there either.
That's pretty much it!