I’ll try to relate my experiences. And if I sometimes comes across less than clear – please pardon my bad English (even worse that my bad Norwegian).
The Behringer DCX is a pro device, and is easier to implement with pro equipment. The levels, gains and interconnects match. With consumer equipment (which I (maybe wrongly? ) assumes that most of us use), it is another matter.
Many of the adverse comments on the Behringer is about noise. IMO the hiss from the Behringer can be objectionable. For the average user – with consumer equipment – it is important to realize:
1) for best results S/N-wise, the Behringer really needs a hotter signal than most ordinary preamps can provide. Volume control in a preamp in front of the Behringer worsens the situation.
2) for best results, you really need some sort of attenuation between the Behringer and your power amps. (This can of course be accomplished with input volume controls in the amps – but this method would not suffice for a volume control)
The best way to use the Behringer in most consumer-orientated settings would IMO be to use the digital input (thereby avoiding problem 1), and to use a 6-channel volume control between the Behringer and the amps.
The gain controls on the Behringer is “inside” the ADC/DAC, and work digitally. This means all attenuation corresponds to a bitloss. An attenuation of 30dB means 5bit. The unit uses 24bit so with an attenuation of -30db you only have 19bit left for the signal. IMO, this bit loss is audible, especially at higher frequencies. The sound gets a bit harsher.
For myself, I therefore in practice use passive resistors/L-pads on the most efficient unit (typically horns) to avoid this potential problem. I only use the attenuation on the Behringer for fine-tuning. This also has the added convenience to lessen the noise/hiss. For me, this is a good compromise.
I still actually recommend the DCX, even though it is a complex device and not easy to use in most settings.