As far as replay gain goes, it is a calculated field and you can either accept it wholesale or delete the individual field so that your default sound level is used. However, there is another and better way to approach this.
Try the plugin EQdb in the third party plugins page. This little app was written by Scot Thomson, president of ThomSound. His purpose was exactly what you stated. With this you can adjust the EQ as well as the preamp function, which will give you an offset to your calculated replay gain setting.
The plugin is still in beta testing. I have found only one bug, which is that if playing now is empty, your first track must be added via the right-click menu, and not via drag-and-drop. EVerything else functions as expected.
This plugin works irrespective of your other settings, it will add or subtract from them. IMO, this is core functionality for digital audiophile systems.