From what I can see and read in the user's manual (PAGE 5), this amp has a built-in pre-amp. It has a direct wired input for CD and also a pre-amp output (from it's pre-amp). You could use another amp, but would need one with it's own built-in pre-amp. If this amp didn't have a pre-amp, it would most likely not have it's own volume, bass and treble controls. Those functions occur in the pre-amp stage. Also, dedicated power amps only have one input - that from the pre-amp. Your amp's Pre-out connections are for a subwoofer, but I ASSume the outputs are full-range and not low-pass only. I could be wrong. If I'm correct, then this output can be used for another dedicated power amp, because the volume and sound controls will work with that output.
What source do you want to connect? Is the reason you want a pre-amp because this one has few inputs? The CD input for example, can be used for just about anything except a turntable.