I use this page to test my earbuds. At the bottom of the page the the binaural test the door knocks should be wide apart, if they are just close to each ear there is a problem.Is there a way to do a test without opening the shell? How about setting the source to mono and comparing the result using the tone generator in my signature? I have a handful of earbuds that sound like ****, maybe out of phase?
https://www.audiocheck.net/soundtests_headphones.php