well first of all, the L/R outputs are fairly quiet. a lot quieter than the headphone output.
second of all, you need to check the back of the synth and make sure that the 'low/high' switch (next to the output jacks) is set to 'high'.
and also, it could be a matter of having patches with low oscillator volume settings. (check parameters 13 and 23.
let me know if any of that helps.
otherwise, get a preamp... or just turn the 'trim' up on the mixer channel for the dw8000.
-r andrew scott.