It sucks when you are sure that the music that you are playing is not being much of a disturbance in the neighbourhood and they still complain about such.
That being said, if you want to stay the good guy (I wouldn't by the way), I will suggest you make use of the sound level meter app installed on your mobile device such as the Sound Meter that is available on both Apple Store and Playstore or make use of Noise Meter which is good for iPhone users. These apps can help detect the sound level when you play songs.
You need to take into consideration the time of the day when you are doing this because it is a key factor since nights are calmer than during the day.
The options above are if I wanted to be the good guy which ain't in my dictionary. So, assuming I am in your shoes, I will first of all try talking to these guys about why they should mind their business and stop worrying me about whether the sound from my room is high or low.
If they don't stop and keep threatening, I will have to inform the property manager letting him or her warn my neighbours about what has been happening and hopefully the property manager gets to handle it in the best way to avoid issues in the future.