The problem is more fundamental than mixing in a studio vs. live PA. It's in the very first question of the OP's post:

"Is it 'better' to mix all the instruments at the same level (90)? "

Until this is answered, it's really a it pointless to discuss mixing studio vs. live sound.

Mixing for studio is significantly different than mixing live sound where depending on the venue and PA and individual instrument and/or amp levels, some of the mixing to the house occurs by proxy, mostly out of the hands of the mix engineer unless he/she is particularly persuasive and/or has control of the drummer's 'energy'.

But the fundamental first question needs answers before a big debate about live vs. studio mixing.

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