The pre-amp is as the name implies the first step in amplification. It takes a very low power signal for a source component and steps it up so that it can then be further amplified by the power amplifier. The reason a good pre-amp is important is that a bad one can add noise or "color" the source signal and then those unwanted additions are passed on to your ear through the amplified signal that comes out of your speakers. Today however, the output signals from numerous high quality sources are strong enough to not need pre-amplification hence the advent of the passive preamp which does a number of things such as allow you to switch sources and adjust channel balance but it performs no pre-amplification rather it just passes the source signal on to the amplifier to do its job.
I don't know much about the Pioneer receiver you have but typically receivers are a set of compromises to deliver a lot of functions in one box and at reasonable price. Some receivers have the reputation for strong amplification sections, others their tuners or decoders. The conventional wisdom is that separate components provide better performance but a number of high end integrated amps have been very favorably reviewed. So I think the test is in the listening, you have some good front speakers and hopefully an equally good center I wouldn't be surprised if you could do better than the sound from your receiver.
I don't know much about the Pioneer receiver you have but typically receivers are a set of compromises to deliver a lot of functions in one box and at reasonable price. Some receivers have the reputation for strong amplification sections, others their tuners or decoders. The conventional wisdom is that separate components provide better performance but a number of high end integrated amps have been very favorably reviewed. So I think the test is in the listening, you have some good front speakers and hopefully an equally good center I wouldn't be surprised if you could do better than the sound from your receiver.