it's will work like you said if "flat transfer" used for already done master which has issues from previous mixing/mastering.
with multis you'll have different story. there is not such thing as not enough bass, guitar, vox, etc. on the track where
those particular part was recorded. so,in this case you'll have more/less discretely recorded vox and instruments and
that's will be your call in regard which volume level should be for this or that parts of the future mix, before blending
them together into single track.
usually presently used techniques involves two stages. first - mixing multis into final mix and second stage - sending this
mix for final touch-up by the sound mastering engineer. SW has well developed feel for the sound, his latest works is quite
a proof of this, thus he just does merges both stages in one by tweaking the sound during mixing stage.