Just took over regular mix duties for 12-piece band (w/ keys, violin, and a horn section) as my friend makes the move from sound to drums. I've subbed several times since he's been running sound for this group and have helped him convince the group to upgrade some key gear like the mixer which is now a Soundcraft GB8 32ch.
Now that I'm running sound I'm thinking that their speakers are the biggest obstacle to them sounding as good as they could... and have to go. (Better mics for the horns are second on the list. My friend becoming the drummer solved their inferior drum kit/mics issue. lol) They currently have a pair of Peavey Subcompact 18 subs and SP-5 15" tops and use a pair of pre-Club Yamaha 15s as secondary mains for odd-shaped rooms. Lead singer has IEMs but everyone else is on older mostly Peavey wedges. Their amps are also nearly 20 years old and so I'm thinking powered tops and subs and eventually powered monitors as well.
Although they dipped into the band's bank account for the mixer purchase last winter I'm told that's a rare occurrence. To upgrade the mains I think the musicians could be convinced to play an upcoming wedding reception for free... giving me the resulting $3000 to start the upgrades.
FYI - I'm not opposed to buying used if it'll get them more upgrades sooner than later... and if the gear is reliable and/or has good manufacturer warranties. For my own benefit, I'd like to replace their snakes with a nice 32x8 EWI or better reel snake... but that can wait.
EDIT: My personal opinion is that with so many vocalists, violin, and a horn section that this band is not well served by 15" mains and I'm leaning toward 12s... or maybe even 3-ways.