Get some music made on acoustic instruments: piano, strings, horns, drums (not drum machine). Maybe a Norah Jones album, or a classical orchestra, or rock from the 1960’s. This is because acoustic sounds are very complex, and you have an idea of what the real instrument is supposed to sound like.
Put the same file on both units.
Make sure the volume is the same as you play them back.
Borrow some better headphones if you can, but even using yours should be OK in a quiet place.
Then listen carefully. Which sounds more realistic? Which sounds richer? Which has more depth? Which lets you hear the individual instruments more clearly? Listen especially to a piano or a human voice or a saxophone, because those sounds are rich enough to reveal flaws in the playback.