I am not sure about this. I think it would be nice if we knew which mixer you have and where the music channel is going in.
Here is what I do:
1. I set the computer volume to barely twiddle the peak light on the input channel with the trim knob (not fader) being mid-low center scale.
2. I set the speaker input gain knob to midrange, as is specified in the manual. I don't know how you can adjust the peak on the input with my Tapco Thump pair (or my Behr. B212 pair, for that matter). They both say midrange is designed to operate with a +4dBu signal.
3. I set the faders on mic and music to about a -10 dB starting point.
4. I adjust the main volume level to please.
5. I ring out the room with an RTA or my ear, whatever is available. On regular gigs, this step is skipped and presets are made.
6. I now can use the faders to adjust the vocal mix or the music mix from track to track with some easily available headroom. As the room fills up, I adjust the mains out, though I will of course crank some songs a bit.