After some checking around the forum I installed Rocket Player (because of the preamp feature) on my android phone. The plan was to play my subs using RP while playing regular music using the default music app. However whenever of I have my subs playing and try to play music (or vice versa) it stops the other app. Meaning that both cannot be played simultaneously. Does anyone know any workarounds for this?
Cheers
Donwload MX player. You can change the settings so that it keeps playing while listening to for example music from Spotify or YouTube.
Musicolet works well for ultrasonic subs running continuously underneath music
Mx player seems to be for video and not audio. Or am I missing something??
it’s also for audio
Eddit: Settings>audio>check the box audioplayer
Okay so I did that, I have the volume not synced to system audio, and I changed the pip/background setting from “stop” to “play in background”.
But when I try to play music in the other app it still stops the masked track 
The “play in the background” is for whether or not you want to keep playing the audio when you have turned your screen off. Instead you need to uncheck the box “play alone”. For me it’s a few options above that pip/background setting.
It’s not necessary to not sync it with the system audio, however it gives you more control when you are using multiple music players at the same time, so it’s a good choice.
Thanks so much for all the help! It works 
Great, and no problem 
For anyone that comes across this thread wondering the same thing. I found a similar feature in the Rocket Player and AIMP settings. This works a little better than MX Player as I couldn’t find a way to make playlists in MX Player (for stacks)