Actually, did you know that Slappy has high level inputs? OK, we won't go any further. :)
A sub like the various REL S and Q series have both high and low level inputs to accommodate stereo and home theater. The recommended set up for RELs in a two channel music system is using the high level inputs of the sub which are fed by wires hooked to the speaker terminals of the 2 channel power amp.
So no, it isn't mandatory that you use a preamp with an active sub, but that really depends on the sub you're using and if it has the same input choices of the REL.
In a home theater set up you can use the LFE output from the processor hooked to the low level input of the powered sub.
The other choice I'm sure you're aware of is having the powered sub receive a signal directly from the 2 channel preamp by way of its own set of outputs if the preamp has four or by using a y-adapter and splitting the 2 channel signal at the back of the preamp. Some people even place the y-adapter at the inputs of the 2 channel amp and split the signal there.