I've listened to a few of the RG models and my opinion is save your money and get a DBX 3BX III. These beat the Pioneers by leaps and bounds. At least that has been my brother's and my experience.
You will find though that as your system improves the need for one becomes less. That's what happened with my brother and I. We reached the point that we are happy enough with the sound of our systems that the DBX units are retired to the closets.
I have an RG-2 that I use, particularly with a local FM radio station that flattens out the dynamic range on their signal. Set correctly (I tracked down a copy of the owner's manual, and don't have it maxed out) it does the job.
When I was using the 3BX i would set it so that no more than 3 led's would light up on the expansion or upper half of the display. Any more than that seemed like to much for my taste. I spent a lot of time listening and determined that was the best setting for me.
So, you can't really have an "I'm going to expand by three bars" rule of thumb... it's like saying "I'm going to set the temperature on my thermostat to raise the temp 15 degrees above outside ambient"... in the winter you'd freeze and in the summer you'd swelter although there would be the odd day it'd work perfectly.
25 years ago the sound level of the typical CD averaged -18dbfs but this level has been rising steadily. Good-quality releases are still in the -15 to -20dbfs range (-20dbfs is the THX standard for movie soundtrack recording... they WANT the impact headroom inherent in a large dynamic range in the theatres). This means the tracks could potentially use up to 40dB of the 96dB of dynamic range available from the CD medium. Today's CD's are mastered so that the average is around -9 dbfs, with some releases as low as -6 to -5 (pop, hip-hop, contemporary R&B, rap). On these pressings, you are only accessing a measly 10dB of dynamic range, the other 86db available is "wasted". These are the CD's that need expansion.
The amount of expansion will necessarily depend on the source material. A well-mastered classical CD will require none, whereas a compressed pop CD will require a great deal. I was listening to the soundtrack to "Beyond the Sea", the Bobby Darin story, the other day and put the expander into the loop. The vocals and drums had SO MUCH impact it actually startled me... it was too much and it made the music sound unnatural.
25 years ago the sound level of the typical CD averaged -18dbfs but this level has been rising steadily. Good-quality releases are still in the -15 to -20dbfs range (-20dbfs is the THX standard for movie soundtrack recording... they WANT the impact headroom inherent in a large dynamic range in the theatres). This means the tracks could potentially use up to 40dB of the 96dB of dynamic range available from the CD medium. Today's CD's are mastered so that the average is around -9 dbfs, with some releases as low as -6 to -5 (pop, hip-hop, contemporary R&B, rap). On these pressings, you are only accessing a measly 10dB of dynamic range, the other 86db available is "wasted". These are the CD's that need expansion.
So, you can't really have an "I'm going to expand by three bars" rule of thumb... it's like saying "I'm going to set the temperature on my thermostat to raise the temp 15 degrees above outside ambient"... in the winter you'd freeze and in the summer you'd swelter although there would be the odd day it'd work perfectly.