We offer stem mastering on request. This should be seen as a problem solver, and not a standard way of delivering mixes for mastering. Export the mix in stems or instrument groups: e.g. drums, bass, rhythm guitars, lead guitars, keys, vocals. The sum of all stems played back at unity gain (“0” on the fader) should sound as your mix (without any mix bus compression you may have). Always include the stereo mix for reference.