Is my question even valid? Like does quality get affected depending on USB vs Bluetooth Audio, etc?
As for me, I'll tend to listen to bass-heavy music (hip-hop, rap, dubstep, drum n bass, etc) and I find bass +6 is "good" but +8~10 would be the bestBut when I turn the volume knob to 15 or higher, the music quality tends to get distorted...probably because my treble is +5?
I could be wrong, and our music styles differ greatly, but I think I read some folks saying the fake engine sounds piped through the stereo will cause distortion when set to the higher settings. Did you try turning that to off?

Mine isn't great, thus the +1 treble setting. Carry on; whatever works for you, I am glad the HK can provide! 

