“I’m sure you’re not aware, but recently we can hear the bass from your music in our unit, and it has been disturbing our sleeping and TV viewing. I may be somewhat overly sensitive to sounds, but I would really appreciate the volume down after _______________ pm. Thank you for your consideration.”
We had an (different) issue with upstairs neighbors in a previous condo building we lived in. We did not know them at all. We wrote a note to them pseudo identifying ourselves (“we live in the unit below you”), that was similarly worded to what I wrote above. Problem immediately solved.
I get the challenge of saying something F2F. But, that might be better since you do, sort of, know them. But, if you don’t want the F2F, “confrontation,” write a note.