This little wrinkle could be relevant.... If the REAPER master level fader is set to +4dB for example: 1) That 4dB boost is part of what you hear when listening to the unrendered project tracks. 2) The 4dB boost affects the level of a rendered project file, boosting it by 4dB. 3) If you ticked the check box to add the rendered output file to the REAPER project, and then solo that track and listen to the rendered file in REAPER, without touching the master fader, then what you hear is boosted by 4dB+4dB=8dB! The master level fader boost is doubled up - once in the render process and then again during playback.