"Is it 'better' to mix all the instruments at the same level (90)? "
I mix all our backing tracks to a nominal -6 dB. Does '90' refer to 90dB? (thanks for the plug

) Is that at 1 meter, 10 meters, 30 meters? Or does '90' refer to the level in BIAB?
That is -6dB
peak, and different tracks will vary greatly in
RMS. Hence the mixing through the PA.
Just my $.01