IMO it's best to use EQ wisely to get a full sound with lower volume.
Each instrument needs to carve out its own sonic space:
eg. roll off lows on guitar and keys = hear bass clearly, a bit of 1.6kHz for articulation,
plus decide between guitar and keys who has a warmer mid-focussed tone and who has a brighter tone.
eg mellow acoustic guitar and very bright grand piano setting, or twangy SC elec gtr (strat/tele) with Rhodes/Wurlitzer/elec piano setting, etc.
Good speaker choice/placement makes a big difference too. No amps on stage only small wedges forces you to be clever with sound.... ie you tell others to play quieter if you can't hear yourself. Takes skill and good communication.
Everything thru FOH gives a blended sound without phase issues from stage sound.
