You can always vary the volume via your preamp but you would be changing the analog signal level into the Behringer DCX 2496 crossover so you could lose bits in the process as this is done before the analog to digital conversion in the unit.

The better way to go is to have digital volume control on the 6 outputs from the digital crossover. Second best would be to vary the 6 outputs after the DAC within the processor. In either of these cases you would have full line level inputs to the digital crossover and you process the digital info without losing digital data. For example, the DEQX has its volume control after it does its digital magic. Some of the other digital crossovers may have this capability but I know that the Behringer does not. The other part of this issue is what happens with sources with digital inputs into the crossover? Having volume control in the digital world or post digital processing is a good thing.

Jim