Hi Quadzilla Dan, I am assuming that you know what ReplayGain means and how it works.
You can read there ...
Add Album Volume Increase via Replay Gain Tag
... about an attempt to manually change the calculated values and their caveats.
For me it is incomprehensible why anyone wants to manually change the ReplayGain values, after they have been calculated.
DD.20111201.0740.CET