That doesn't sound like a gain problem to me. After looking at the specs and measurements for several of these products (the gain of your PL3, btw, is either about 10 db or about 12 db per John Atkinson's measurements in Stereophile, depending on whether it is the Prologue or Dialogue version), and after doing some calculations, I'm thinking that what might be happening is that at the 3/4 volume setting you are maxing out the voltage swing capability of the PL7s.
Keep in mind that in a passive biamp configuration (i.e., without an active crossover "ahead" of the amps, which apparently is what you have, aside from a high pass filter that may be associated with the Entec sub), both the high frequency amp and the low frequency amp have to output voltages corresponding to the full frequency range of the signal. So the PL7s in your setup will have to output just as much voltage as the Moscode, while presumably having a much more limited maximum voltage capability.
If that is the issue, the PL7s will be clipping on high volume peaks with the volume control turned up beyond that 3/4 setting, especially with the CD sources. Perhaps that clipping is soft enough, though, that you haven't perceived it as such.