If you plug in your sound output to some external home stereo equipment, you set your volume with that equipment, not with Windows. So you can set Windows to 100%. But the Windows master volume is nothing OBS-related. OBS grabs the audio signal before it reaches the master volume setting, so relevant for OBS are the individual per-app sound settings from the Windows mixer. And, if such a setting exists, some individual sound volume settings within an app. Games usually have such a setting. You need to set these to maximum to let OBS get the maximum.
Summary: You have 3 Windows sound settings: within the game, within the Windows audio mixer for the game (and every other app), Windows master volume. You need to balance the first 2 settings, and additionally the per-source volume within OBS.