Try this:
Take any 2 songs that have very different loudness levels.
Use WACUP to scan them with ReplayGain and write the gain information to their tags.
Play the songs in WACUP.
Then play them in Foobar2000.
My experience:
The difference between the how the 2 players output the songs is stark.
With WACUP, I still got a very soft and a very loud song (in terms of volume levels). I actually couldn't tell the difference in the loudness before and after ReplayGain.
With Foobar2000 the played levels are more or less equal (for the songs).
The songs I used were these (I downloaded and converted them to .mp3 with with yt-dlp and FFMPEG. Both are "All I ask" by Adele, one is a cover by Bruno Mars):
1) Low volume levels song:
https://www.youtube.com/watch?v=oCtYekG1qBc2) High volume levels song:
https://www.youtube.com/watch?v=sonzlE32YVgI've tried this with multiple versions of WACUP. I'm currently using the latest public release. My settings look like this:

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