This is true. Remember the old Rockford Digital Sound Processors? Those were some of the nicest pieces of equipment to ever come out of the Rockford family and they only used ONE set of RCA inputs. This was the highest end stuff you could put into your vehicle and all the show winners had one. The DSP seperated out all the signals coming in through the one set of inputs and divided it up to several outputs that were completely tunable and adjustable.

Now that DSP equipment is pretty much obsolete, you don't see this much anymore. But, most good active crossovers only use front and/or rear RCA inputs now anyway (as was aforementioned). A good active crossover also has adjustable pre-outs that can go from .5v to 6.0v or so. Makes it a breeze to get rid of any unwanted noise in the signal.