Don't know if this will help or not. But I have run powered subs the same as you mentioned on previous machines. I recently bought a met led pro and hooked up a sub and had the same sound issues you describe.
In the game menu I changed the music attenuation (or something like that) and speech attenuation. Not in front of my machine so I don't know if I made it -5 or +5 to increase the music and the opposite for the speeches. My machine jams now and the sub really kicks. Hope this helps