This userscript makes a button that adjusts the gain of Youtube videos automagically because Youtube does not amplify quiet videos, it only quietens loud ones.
This should lead to a much more consistent loudness level between videos.
You can download the script here: https://github.com/ilman01/Youtube-Loudness-Correction/raw/master/youtubeLoudnessCorrection.user.js
If you are using tampermonkey, it should pop-up with the install page automatically.