oops, I was thinking in terms of how I use it and fumbled the explanation Sorry. :)
Just to make sure I remember correctly. The PCK4 is not a unity type expander, correct?
Now that I think about it, maybe I used the percussion strip's expander instead of the PCK4 to help out bringing up soft passages before a compressor to keep things from sounding overly compressed. Gah, I think my middle age brain is getting a bit rusty. :/
Note: A unity gain expander will not affect levels above the threshold. Where as a non-unity gain type does.
The idea is that instead of raising the over all level of the single with the fader which will also bring up the normally inaudible parts (noises from dampening the strings with your pick hand when your not playing for instance), you would use a non-unity gain type expander to selectively bring up the louder portions of a low level signal without dragging up the noise with it.
Optionally, follow the expander with two or more parallel compressors that are mostly dry signal. The cumulative dry+wet sums of both compressors (with the expander controlling the feed into the first compressor) would bring up the levels in a more natural manner.
I am assuming that the peak levels of the quieter potions of the recorded track are low, which is why I suggested the expander (i.e. not transients to cause clipping issues). The Op can use the expander via automation to bring it in and out during the quite portions . A compressor may make the track sound too different, and bring it in and out will sound too obvious and unnatural.